So, I re-installed Windows 7 recently and tried re-installing Pro Tools SE 8.0.3a (disc came with my Fast Track interface that i bought last year). After I select my Fast Track for the second part of the installer, I get a Runtime error. When I click OK, the installer tells me it's finished anyway. Then, when I try to launch Pro Tools, it won't recognize my interface (I guess because it failed to install). Does anyone know a solution to this? It always worked fine before I re-installed Windows...
"Most people work just hard enough not to get fired and get paid just enough not to quit." - George Carlin
Reinstall everything and try again in this order:

1) If you have a disc with drivers for the interface, install that first.
2) If you don't have the drivers on disc download latest from their site, make sure you're running the same version of Windows as your drivers, there are 32 and 64bit drivers.

3) Run installation of Pro Tools. If you can right click on the installer and select "Run As Administrator" which will elevate the process and hopefully complete correctly.

There is a possibility that there are some patches and fixes that have been released after your purchase, so it might be worthwhile checking if you can update but I'll suggest doing that after you see that the installation fails again, since you had a working version my guess is that if failed because your audio card wasn't installed properly and Pro TOols needs to find the audio card to register the software against that, one of their nasty little quirks if you will...
Well the interface drivers are on the same disc as Pro Tools. In the installer you can choose between several devices but I just get the error when I choose mine.
"Most people work just hard enough not to get fired and get paid just enough not to quit." - George Carlin
Sounds to me like it is time to contact Pro Tools tech support.

It is possible you updated your OS and that's why the new install won't take, I had my Mac do one of these quirks when I put in the latest updates and broke the installer.

Sometimes there are library files that are left over and you need to manually clear but my suggestion will be to check the Pro Tools support site with your particular type of error.