if you buy a 2nd hand one, just google and check that you can still get drivers for it, ive got an old edirol external sound card, after 13 years it is still in %100 working condition, however they stopped driver updates for it at windows 7.

The best advice I can give is if you want to record audio with any sort of quality and no hassles--- Buy a proper audio interface. 
They start at around $100 and go up. 
Personal experience with Creative sound cards-- I wasted 2 years of my life fighting to get it to work. 
Your tracks will be out of snyc with each other as well. 
The problem is Creatives ASIO driver is very poorly done. Try a loopback test if you don't believe me. 
 
https://www.centrance.com/downloads/ltu/  
 
Here's my blurb on Audio interfaces. 
http://www.cactusmusic.ca/Recording.htm
 
 

"All my gear is going into my mixer then into the computer via the Line In port on the sound card"
 
You mention that all your instruments go into a mixer and then into the line in of your audio card. Do you run the output of your audio card through the mixer as well? 

I think reginaldStjohn may have hit something. If you are routing everything back out through the mixer then you may be experiencing a loop somewhere in the mixer that is adding to your newly recorded track.

I found the full name of the card I'm using, it's a Creative Sound Blaster X-Fi Titanium PCIe Fatal1ty Champ Series. I paid $200AUD for it in 2013 so it's 5 years old now, maybe it needs replacing? 
 
There is no routing back to the mixer (unless it's happening in the software somehow?), the only input to the soundcard is through the soundcard's "Line In" jack (from the mixer) and out to the speakers through the speaker jack.
 
Sadly I have no old files to test as they all got lost in a total meltdown of my last computer. 
 
I've disabled everything in the soundcard except the line in and the speakers out, no digital buttons are activated - it's still recording from other active tracks even though the target track is the only one with the record activated. If I mute the other tracks they don't record which does lead me back to wondering if there is a "What You Hear" function somewhere that I haven't found. 
 
I was going to reset the preferences in case I messed up something there but there doesn't seem to be an option for that. I could try uninstalling and reinstalling Sonar in case there's a bug or something going on. I can't find it in the uninstall programs so guess I'd have to uninstall the Command Centre and start fresh?
 
Cactus Music: Thank you, that's a lot of great info there and on the linked page!  
 
Thanks to everyone, I really appreciate your input. 

What you have is a really good soundcard for listening to you tube and media on your PC.
What you need is a really good soundcard for recording music.
 
In order to have multi track layering (record each instrument separately on its own track) you need multiple inputs. you don't have multiple inputs. You have one input (your line in on your soundcard).
My advise to you right now is to record one instrument at a time and layer them.
Its really your only option. Most but not all newer soundcards (for recording audio) have 2 inputs.
So you can record 2 instruments at a time and they will go to different tracks (no bleed).
Even 90% of mixing boards with a USB plug don't have multi track compatibility's.
 
Build a song from the ground up:
1- Start with drums. its dictates the tempo and prevents drift.
2- add guitars with main melody (I typically do bass)
3- record Bass tracks
4- piano
5- vocals.
 
It has to be done in phases if you want to have everything on separate tracks.

guitaruke
Input: Stereo Creative ASIO Mix FL (does it with Mono too) - ASIO Mix FL is the only input that works.

This is 'WhatUHear' by another name. 'Stereo Mix' is the output sigal, so you're re-recording the playback of existing tracks via an internal path in the soundcard. The input needs to be set to something like 'Line In' or 'Mic In'.
 
If you don't see those inputs available as a track Input, then either they aren't enabled in Audio preferences or the driver isn't exposing them to the O/S or there's some another setting in the Creative software that needs to be changed. If this was working previously, you might need to reinstall the driver. If all else fails try WDM driver mode in SONAR instead of ASIO.

brundlefly
 
 
This is 'WhatUHear' by another name. 'Stereo Mix' is the output sigal, so you're re-recording the playback of existing tracks via an internal path in the soundcard. The input needs to be set to something like 'Line In' or 'Mic In'.
 

OMG brundlefly - you nailed it - problem solved!!!   
 
You were spot on about the Mix FL being another name for What U Hear...I had no idea. I found a driver setting that allowed for "Creative ASIO Line /Mic /Aux L" and tried that one instead and voila - I can now record each track independently with zero bleed while still being able to listen to them as I'm recoding. It's all working perfectly now and I can happily get on with some recording. Thank you so much! 
 
Thank you to everyone who replied, I really appreciate your taking the time to help out and I learned a lot from your detailed comments. 

Late checking back in, but glad to hear it, and glad to help. 
 
Cheers,
Dave

Please change your thread title to start with the word SOLVED if you get a moment.
 
then people that see the thread in a search will know they can go here for a possible solution to a similar problem.