There is a difference between OS slow, and Video display slow.
Usually, the OS slow is caused by running out of available memory and having to swap to the page file on the hard drive to load up programs.
Video slow in Window$, would be: you start a program, border of the program window is displayed, but nothing within the window itself is displayed yet, then, either piece by piece or in a unrolling window fashion, the program itself finally appears on your screen. This can also be caused by having to swap to the page file though. A good way to test this, is to load a very large image at its full size, and scroll the window all around. If the video card isnt up to the task, there will be lots of tearing, very rough scrolling, missing parts of the image for some time while the display catches up, etc.
Personally, the system specks you give, I dont think the video card is the culprit. Do some spring cleaning on your system, especially defraggin the drive(s). And if that doesnt cure the slow, check to see how much memory your system is using. To do this, open Task Manager, goto the Performance tab, and look at the Commit Charge box.
Total is the amount of memory your system is currently using.
Limit is the total amount of memory your system has, ram and page file together.
Peak is the maximum amount of memory your system has used at any point in time since reboot.
If the peak exceeds your total installed Physical Memory, or is constantly near that value, you need more ram. You want to avoid swapping completely, although there are some things that Window$ will swap even if you have more then enough available physical ram.
Other suggestions; put the swap file on your least used/faster drive seperate from where the main OS and programs are installed. Close unnecessary programs and review all the programs which are loaded and stay resident even though they dont really serve a purpose (lots of companies out there install useless programs that wont affect the main program if thier not loaded, ie Adobe and thier quick laucher program [like >1 second faster loading of Acrobat will be noticeable...]). You'll need to check the Startup folder, as well as the Run portion in your registry (you can also type "msconfig" and disable programs that are ran at startup that way too if you dont know how to fiddle with your registry). You can also
check out this article and disable unnecessary/unused services on your system. Run windows with the Classic shell and no eye candy enabled instead of the modern XP theme with all its eye candy (this is really useful if you use the HTPC all the time with a media type frontend and dont goto the normal Window$ desktop at all).
If I think of anything else, I'll post it up later.
