In regards to Bull Guard, it's not in this list of rogue anti-spyware (http://www.spywarewarrior.com/rogue_anti-spyware.htm), but I'd be leary of any software of this type that you have to pay for as there are several excellent ones available for free.

As for your problem, my first thoughts would be overheating, RAM (not enough or going bad), or virus/spyware/adware.

Have you cleaned inside your computer lately to ensure there is not a collection of dust inside? Be careful when doing this and wear a wrist strap to prevent damage via static electricity.

How much RAM do you have? Is your computer showing the same amount as what is actually installed? Right-click on My Computer, click on Properties, on the General tab the amount of RAM should be shown near the bottom.

To rid your system of intruders, follow the steps in this thread:
http://www.daniweb.com/techtalkforums/thread5690.html

If you need to post a HJT log (described in that thread), do so in the Security forum along with details of any problems you're still having.

Check you processor fan and the power supply. Your Hard Drive might be taking a dive on you.

I am having the same problem exactly; I also coincidently have a Compaq Presario running XP.
I have recently upgraded my RAM to 2Gb and I don't believe its an overheating issue. Strangely, although not totally sure, it seems to happen at the same time of day each evening plus random other times for no obvious reason or special activity.

I use AVG for virus and Spyware scanning and there is nothing showing up.

This is getting to be a real pain.

Any ideas?