fix computer errors

Well, i have:
Intel P4 3.0Ghz(HT enabled)
1Gb of RAM(Motherboard supports up to 2Gb)
nVidia FX5500 256MB Memory
3 HDDs:(please ignore the dots)
…Seagate IDE 160Gb (50Gb for Windows XP-frequently used & 110Gb for data)
…Seagate SATA 120Gb (for Data)
…Maxtor 40Gb (for Windows Vista)
…(Therefore, i have 4 drives in “My Computer”: C:, D:, E: & F: )
Motherboard: Asus P5V800-MX(supports Intel Pentium D)
1 CD-ROM
1 CD/DVD+-R/RW
1 expansion card installed(Winfast Global-for recording videos)
I am a massive internet user(I download alot), multi-tasker(surfing the net while copying files, downloading, printing & etc.), data keeper and absolutely NOT a PC gamer – I use my computer for personal use, education and sometimes business. I edit pictures at 3k-20k pixel sizes, create HD videos, making flash, edit sounds, etc.
While watching Eagle Eye movie(copied the file from DVD to HDD) on my XP, my computer restarted on itself. After startup, it showed “recovered from a system failure. A log file has been created” why is that?. I restarted my PC again but, now, it takes more than 30mins loading the startup. I restarted my system again, but this time i used the Vista do some check disks and defrags. I still get the self restart while watching Eagle’s Eye on Vista. I suspected that there should be something wrong. I checked the CPU fan and temp, they’re: 1950-2000RPM and 42C-51C(are these values normal?) or have i just loaded lots of HDDs? or should i need upgrade system: buy new CPU, add more RAMs and buy higher display cards? or is my OS corrupted* and needs a reformat as well as installation?
*Recently, XP showed an error about Dr. Watson(what this?) and then laggs. No errors in my Vista.
Any suggestions, explanations, & tips on how to avoid sys failure? Tnx alot!