Slow computer and sudden shutdowns! Please Help!

Hello....
I am totally new at this, only had my computer for about 5 months...lately though i have noticed that it slowed down considerably then so I ran all the spyware proggs that have been recommended, followed the spyware & trojans (from major geeks)tutorial.......I downloaded spyware doctor, spybot, ad-aware, spyblaster and a-squared, plus i have the trial version of the Spy Sweeper. I'm not sure why, but after i did that my computer became even slower and for the past couple of days it suddenly shuts down while I'm doing work on it...it doesn't happen at the same time everytime, or when i open a particular programme...i could be doing anything it would just shutdown.....I ran a Hijack this! scan and then followed the tips in the Hijack this! tutorial from MajorGeeks. But i don't know what else to do....it keeps happening....is there some sort of programme clash?? I have Nero, PowerDVD.
My system is running on Windows XP prof, service pack 2. pentium4, CPU 1.6GHz. Anything else you might need 2 know in order to help please let me know!!!
Thank you all