Never had this before

Hi, The past few days I have been having problems I have never experienced before. Symptoms include the whole system freezing, Internet exploring coming up with an error message that it has encountered an error an cannot continue, programs failing to open such as Incredimail, Spybot freezing during a scan, The occasional Blue screen of death.
My AVG anti virus is up to date, I have completed scans with spybot (though not always) and it has reported nothing. I have defragged. I was thinking maybe this was a hardware fault but the only spare component I have is an old Maxtor 40 gig hard drive which I replaced and the strange problems are still there. Can anyone help me with testing other components such as the motherboard.

Current specs
Motherboard.....Asus K8V SE Deluxe. Socket 754
Primary HDD Seagate 80Gig SATA
Secondary HDD 250Gig SATA
CPU AMD Athlon64 3200
1024mb Kingston valueram
O\S Windows XP SP2

 

Thanks, but I'm not convinced this is not a hardware problem. I want to attack from both sides and hopefully get my machine up and running properly