I've been using Sonar for several years now, and recently switched everything over to a different computer.  I'm not sure how much of this information is relevant, but I'll try to be as thorough as possible to find a solution.

I have an Acer PC, 2gigs of RAM, Tascam us-800 interface, Ensoniq Mirage synth (yes, it's fossil!)

The other night I had recorded a drum midi track, vocal and guitar, and was playing along with the track, trying out a keyboard part, when the computer froze, stuck on one note.  I shut everything down a restarted.

Now, my PC will not recognize MIDI data coming from the Ensoniq.  I don't get any MIDI activity lights. Everything else seems to work fine.  
I hooked up the interface to my mac book in GarageBand and I can play and record MIDI there.

I uninstalled and reinstalled both the interface driver and sonar...still no luck.

I installed MIDI-OX and checked for MIDI activity.  I'm not 100% sure I know what I'm doing with MIDI-OX, but when I open Midi port activity bar,I don't see any lights flash when I push keys on the synth.

So, in short, Sonar recognizes the Tascam interface, can record live sound coming in from mics, but not MIDI data coming in from the synth.

Puzzling....Any ideas?

Did you disconnect the Tascam when you re-installed the driver? USB devices should be unplugged when you install the driver and then you plug it in after the driver is installed or when the installer asks you to plug it in. 

Yes...installer prompts you to do so in case you forget.  I restarted computer as well both after uninstall and reinstall.

Might be the same for all things Tascam, I could never get MIDI to be stable under W xp 32 bit drivers. But the good news is it now works 100% with W7 64 bit. 
I forgot about this until the other day I fired up my Xp 32 bit laptop I use for live recording. Audio is 100% fine, But I went to add a keyboard part to this quick project I was doing and it all came back to me why I never used the Tascam for MIDI with my old 32 bit desktop DAW ,, it's the bad drivers, the 32 bit drivers are just plain bad stuff. At least for the us1641.  

So...the best solution may be to upgrade to Windows 7...or 8?  

I'll have to see if my processor can handle that....

Well, if it was working until now, it should still work. 

Did you check Options-MIDI Devices to make sure its still selected and at the top of the list?

Yes...midi devices lists us-800 selected and on the top.

Tried reinstalling the driver with the previous driver still installed; hoped the new install would overwrite or erase files that the windows uninstall might have missed.

This seemed to work.  Sonar recognized MIDI input...for :30, then it crashed again and will no longer recognize MIDI after repeated attempts to reinstall drivers.

Do you guys think upgrading to Windows 7 64 bit will make a difference?  I've got 2.6 process and 3 gigs of RAM.  (I mistyped 2 gigs earlier...sorry)

This system worked pretty well on an HP with XP with only 1 gig RAM.

Yes for me all my MIDI issues went away when I upgraded to W7 64 bit, I think they might of de bugged the drivers as they must have been receiving plenty of hate letters for the XP drivers. Some found that the XP drivers worked fine under XP SP 2 but not for SP3.  
Now you have a different interface, but I would have a sneaking hunch they use the same code for all their usxxx products with just minor changes to the the code for the ASIO panel. 
Your old acer might not run W7 and 2 gigs of RAM is pretty minimal for it. 

I have this old Toshiba Satilite Laptop from 2004 ( XP 32 bit)  someone gave me . It only has 1.5 Gigs of RAM and a P4 processor. I can't afford a new laptop so I'm going to cross my fingers and hope it works with the Tascam  this weekend. I will only be recording 4 tracks of audio so I don't see it causing me grief. 
One thing about the us1641 is it uses AC power and it has enough juice to feed Phantom power through a 160' snake. My M audio would not do this. 
For what I do it has been a most excellent purchase. 
My wife has a IBM Think Pad i5- W7-64 bit-6 GIG ram- that I loaded with Sonar and the Tascam drivers and it works great,,,, but alas it's her's and not mine. 

Oh and another note, I just read what you said in last post about the crashes, And that is exactly what happened to me. It would work and then freeze everything when ever I tried to use a soft synth other than the TTS-1. And If I tried to record MIDI it just didn't work at all, It would be this huge delay and then crash. 

As long as I used it for nothing but audio it was fine. It would play MIDI files on the TTS-1 but even then the CPU meter would slowly climb and then audio engine would stop. 

So I just put the Tascam aside and only used it with my laptop for live recording's of bands and concerts. I reverted to my M audio which had other issues but at least the SPDIF and the MIDI works fine. 

So...here's the update.  No real progress.

I downloaded and installed windows 8.  However, after more research I discovered that the installer recognizes the architecture of your system then downloads that version.  So, in short, I got a 32 bit version of windows 8. 

So I ordered the backup DVD with the 64 bit version.  Did a clean install, tried to install sonar 5 64 bit version, but near the end of the install it kept prompting me that it could not install dll files, then said it could not finish sonar install.  "contact tech support."

I tried installing 32 bit version, which seemed to work.  Computer recognized midi, everything seemed cool.  Until this morning, I opened Sonar and it won't recognize midi again.

I'm ready to hurl myself through a wood chipper.