So .... my PC has suddenly decided to add about a quarter of a second latency on the sound card line-in socket (Creative X-Fi). It's not sonar causing it because the latency is there even when sonar isn't running. I know a lot of people have faced this problem before, anyone have a solution? I've fiddled with just about every sound setting on the PC, creative apps and sonar, and even re-installed the sound card drivers, but all to no avail. Makes it a nightmare to sequence external synths & drums.

I'm actually surprised the latency isn't much more with Creative's Sound Blaster X-Fi as they're not made for pro audio dedicated DAWs. Are you using ASIO4All or the drivers from Creative? Either way, I'm suspecting that it's this audio card that's going to be your problem. You might want to look into getting a good audio interface that's designed for pro audio (recording/playback) and won't give you these headaches.....(i.e check out the Focusrite interfaces).
 
  

I'm using the Creative drivers. I have actually ordered a USB Audio interface anyway because my line-in is a little noisy. Hopefully will arrive next week. Although you say I should expect latency I haven't had any until a couple of days ago, after a month of using sonar and over a year of using Cakewalk Home Studio 9 before that, all on the same card.

TimDale
I'm using the Creative drivers. I have actually ordered a USB Audio interface anyway because my line-in is a little noisy. Hopefully will arrive next week. Although you say I should expect latency I haven't had any until a couple of days ago, after a month of using sonar and over a year of using Cakewalk Home Studio 9 before that, all on the same card.

That's quite impressive on Creative's part.......Sonar is genereally quite demanding in this dept. and for you to be able to use it this far is remarkable. Of course, I'm only going by all the complaints of people using SB audio cards in Sonar and the numerous issues.
Which card did you order?

The hidden issue with on board and MME drivers is that there is this drift in timing you might not notice. Your audio card is your timing master. And this is where a $5-$45 Audio card is faulty. 
 
Most of us who started using computers 15 or more years ago started out with on board audio. It was all there was. The fancy upgrade was to a PCI card. Good ones where made and some are still in service like the Delta or Motu. But the good ones had proper ASIO drivers. Creative claimed ASIO drivers and I so bought one in 2003. It never did run properly in ASIO mode. It worked best in MME mode, And later in WDM mode via asio4all. I was always fighting with my recordings timing. I'm picky about recording bass as example and had never had a problem using hardware recorders. I blamed it on the software which was Home Studio. 
Midi was not a problem as that gets quantized to the grid but why did my audio tracks sound out?  
I soon learned that each audio track I recorded was not in sync with the others.  It was actually random where they would line up. They also drifted over time, what a mess!  And just by enough that you couldn't see it unless you zoomed way in. Timing clock issues= drivers= audio card
 
I returned to using my Yamaha MD8 hardware and my Atari for midi for another 5 years until the Atari died. I then got smart thanks to the forum here and bought an M Audio interface and all was ,, well it was better and I could now be in sync. So it only took me 10 years to get this working :) 
 
Wait for your Audio interface to arrive. I hope you purchased the one you need.  

the x-fi's are terrible for any serious audio.
move to Plan B.

Equipment status symbol issues aside, the problem is that you were running the current setup with acceptable "latency" and now it has become too long. First what do you mean by latency and why do you say it is on the sound card line in? That phrase would seem to mean that if you input live audio into your sound card, there is a half second delay before it shows up somewhere. Where is that somewhere and what is it going through to get there? Note that your problem description does not even indicate if you are routing through a DAW, let alone what effects etc. might be involved.
 
The usual way that "latency" is experienced is that there is a delay between the MIDI send from a controller until the triggered event in a softsynth produces a corresponding output at your sound card's audio out. If that is your problem, your description does not say so. You need to provide full routing information in sufficient detail so that someone can help you, otherwise all you will get is snarky Soundblaster stink. Blaming the tool is always the easiest, but often not the most accurate answer to a problem.
 

i owned a X-fi card.
i know of what i speak.
 
pure experience,
without the hyperbole

Okay to explain the audio ......
I have a number of external hardware synths wired through a mixer and fed into the line-in socket on my Creative Labs X-Fi sound card in my PC. The output is either on speakers or headphones. I have been using Cakewalk Home Studio 9 for 2 years on this PC with no problems. I upgraded to Sonar so I could use VSTs. That's when the problems started. There is now a noticeable delay between hitting keys on the synth and the audio coming out of the speakers. I plugged the headphones into the mixer with the PC speakers still turned on, and you can hear a distinct double hit of every note. The problem persists whether Sonar is running or not, so theoretically it's something to do with the sound card config. I had a problem with latency when I first installed Sonar so I decreased the ASIO buffer latency from 50ms to 20ms and that cured it. Suddenly now without warning the latency is back, and nothing will get rid of it. The ASIO latency was still 20ms, I tried reducing it further to 10ms but it didn't make any difference.  (The only way I can now use my external synths in Sonar is to slide the recorded notes left by 1/8th bar (at 120bpm), i.e. 0.25 seconds, and then it works. Hardly ideal.)

What I have ordered as a experiment is an external USB audio interface by Behringer (stereo in, stereo out). I will see if this solves the problem (by essentially bypassing the sound card).