(..... not responding)

Had a system failure recently.  Ended up being an overheated video card, but in course of troubleshooting ended up installing a new hard drive and video card (both upgrades).  Made sure new video card was supported by onboard power supply.  Probelm is, since I installed hardware and reloaded operating system/software, the computer has had performance issues.  When opening applications such as internet browser, email, software program, etc., the system will freeze/stop responding for a period of time before 'catching up' and continuing on.  This happens frequently.  I have gone through the system checks that I know to perform, but cannot seem to ID what the problem is. 

Inspiron 531 Vista Home Premium  2.00GB RAM  No third party software installed other than what came with the system, Windows Experience Rating = 4.9

Any help/ideas of what to check is appreciated.

Thanks!