I had a similar problem on my old Mac with an Edirol USB card, no matter what I did it just wouldn't record properly and in my case it was a weird issue of GPU settings, background services and a slow hard drive (5200 rpm) and probably something to do with the USB port speed rate. On the other hand a firewire card worked fine so it might have had something to do with the quality of the drivers. Another thing to try would be ASIO4ALL drivers for your audio card and see if that might fix something. Go highest latency possible, lowest audio resolution you can live with (in my case 16/44.1) and kill as many background services in Windows as you can.
Sometimes lowering your graphics card settings could also help as these GPUs suck down from the ram and motherboard resources to run graphics as well.
Are you deadset on using Windows 7? It might be worthwhile to go down to XP or even Linux in your case as they can render better results. If your audio card has standard USB drivers (my Edirol does) it will be recognized under Linux.