Games/Programs/Mouse pointer Stuttering and Freezing

Question

Hi people
I've recently (5 days ago) built a new PC. I have had loads of problems with it.
First of all after building it, i installed windows XP Professional SP2. Although the PC worked, it was stuttering.
Whenever i performed a task (opening a program, extracting a zip/rar, loading a web page...LITERALLY ANYTHING..) the mouse pointer would pause/stutter/lag for a brief second then return to normal. It would do this several times depending on the length of the process that the computer was dealing with. If i was downloading a file, the mouse pointer would do this until the file was completely downloaded. Sometimes it would simply freeze on the screen and you could see a "Ghost" of the mouse as it rolled over things/selected things.
I even tried doom 3 and the game did the same. A pause every second or so. This also affected the sound causing it to loop sentences (which is why i think it's a mainboard prob.)
I followed the MSI manual and spoke to friends to try to sort this out. I installed all the relevant drivers for the Graphics Card and then went to install the NForce drivers. When i did this and then restarted, Windows wouldn't load anymore. Frustrated, i reinstalled Windows.
Reinstalled, i tried reinstalling them again. Same result. Another reinstall later and i decided to consult the MSI website. It advised updating the Bios. I consulted a computer savvy friend and he suggested the same thing. I did this and then damn thing wouldn't boot anymore. It got stuck on the boot up screen. Very pissed off, i tried reinstalling windows... AGAIN.
Still no luck. It simply wouldn't boot from the hard drive. After 2 days of worrying, i managed to reset the bios back to v1.0 and another reinstall of windows and i was back to the start.
I have left it for now but am incredibly worried about installing the new Nforce drivers or updating the bios again. It was a bloody nightmare.
Now you know my story... here is the question.
"Does anyone have ANY IDEA WHY THE HELL MY COMPUTER SEEMS TO BE FREEZING/STUTTERING?"
My housemates reckon that it could be an IRQ conflict. If that isn't the case, what else could it be? Any ideas would help. I'm not very confident with computer hardware problems and don't know much of the jargon that goes with it.
I've searched the forums and i can't find anything that is too similar although the "USB IRQ Conflict" problems seem possibly relevant. 
Cheers
a very confused/bemused/annoyed Alistair. Ealing, London, UK
Mainboard - MSI K8N "Diamond" SLI (Bios 1.0)
Processor - AMD Athlon 64 3000+
Graphics - Geforce 7 7800GT (Drivers Version: 81.85)
Memory - Crucial 2gb kit (1gb x 2) DDR
PSU - Win Power ATX 450w (Provided with the XBlade Case)
Disks - Seagate 250gb (Master)
       Western Digital 120gb (Slave)
       LG DVD-R/RW 16x Multi Layer
OS - Windows XP Professional SP2
      

Answer 1

I've done a graphic card test and that is ok apparently, I've done 3 FULL formats of the Hard disk and all the drivers that i have installed are all the newest out there and are all the correct ones. As you can also see, i have a 450w power supply which should be adequate enough to handle the system.
Hmm... This is a bugger of a problem. Thanks for the advice though.