I just reformatted Windows 7 yesterday. Prior to the reformat some things were a bit sluggish but Borderlands 2 (and other games) were running fine, for the most part. After reinstalling everything and copying my old steamapps folder, Borderlands won't work. I get to the launch menu, hit "Play" and get a box saying that the program stopped working. A larger window appears letting me know that I don't have enough virtual memory and that I should clean out my HDD.
When I reformatted I neglected to erase my old files. I did this post-reformat. Might that be causing some issues? Maybe it's a driver problem?
right click on my computer and select properties/advances system settings/advanced/performance settings/ advanced and virtual memory. set this to manual and set it to 1.5 times you actual ram... so if you have 4 gigs make it 6 or if you have 8 then set it to 12. set the minimum to 1mb less than your max.
save and quit . you may have to reboot. sometimes windows asks for it, just do it and then you should have no virtual mem errors for anything you run.
i dod this as a basic rule of thumb on every pc i use and have never had any issues with running out of mem...