I recently put together a system for my friend, but he has been having a great deal of difficulty with it lately. He's been having problems with the computer as a whole being very sluggish. For example, his mouse will appear to stutter when he moves it. Whenever he works with Skype or anything video oriented, his computer will slow to a crawl and/or freeze. He needs this system to be working flawlessly because it is his main computer, and he talks a lot through Skype to his family overseas.
I feel back because I built it for him, and when I checked everything out, it seemed to be working fine. I suggested he install the most updated video drivers (hoping to cure the video problems) but that didn't work. His OS is running off an old PATA 160GB hard drive. I don't know if that was a problem, but I told him to listen for clicking noises and see there were any. I was thinking about doing an install on the 1TB drive, but he wants to keep that for storage specifically if he can.
The system consisted of the following:
Windows XP Home-32bit
Case was reused from previous system (old, don't know what it was exactly)
Well, the RAM used in the build isn't exactly a top performer, but it shouldn't be hindering the system as much as you are describing it. Not running RAM in dual channel is another small performance hit. Is he using an old IDE DVD drive as well? Might be running into some bandwidth bottlenecks trying to run the OS off an old drive like that. Sounds like there's video related issues also. the HD 4830 should be fine for his uses, are the video drivers up to date?
Definitely run Memtest86+, there's also a free version of some basic hard drive benchmarks called HDDScan that might be interesting to run. If you're still not turning up much, get an Ubuntu Live CD or something equivalent, and see if you still get poor performance running Linux off the CD. Might help narrow down if it's hardware related, windows configuration, or that shady sounding hard drive.