Whether I use Mozilla Firefox, or Internet Explorer7-8, if I stay on a Website for at least 1/2 hour, or more, sometimes as little as 15 minutes, my Computer will lock up. Firefox is a little better, in that I don't get constant messages saying there's been some kind of error, when I simply close a Webpage, but I still experience lock up, and occasionally, even using 'CTRL-ALT-Delete', won't work to close the locked page, and I have to resort to a hard shutdown. I have run two different Registry cleaners, I have always had a Virus Program on this Computer, even after I replaced the old flaked out Hard Drive 3 months ago, and yes, this is the Drive I ran the Registry Cleaners on. I even ran a Malware detection program, and the problem still persists. Could it simply be that the new Hard Drive is defective?

My Computer is a Hewlett-Packard SR1950NX, running Windows XP Home Edition.

Are you sure the problem isn't just the half an hour and not necessarily your browser?

If you leave the PC on (with no programs running, just Desktop) for half an hour does it freeze up again?

Sounds like an overheating issue. You may want to clean the dust from the case with either compressed air or a hair dryer on very low setting.

If you have had the PC for a long time then you probably also need to reapply thermal paste to the processor. Adding another fan to the case may help but may not be possible especially if your PC is store bought.

Use this program to check your PC's temperatures:

Download SpeedFan - Access temperature sensor in your computer

High temperatures are anywhere from 50C + really.