is it an ACER computer? There was 4.4gig processor model that was recalled a few months ago for "faulty wiring" at the power supply.
from an Electronics point of view, a cold-solder-joint in the power supply sounds like the trouble maker here!
sometimes the connection being make ( that in the Orange area ) has dirt on the wire or Oil on the curcuit board which will not allow the Solder to connect the wire to the curcuit board. ... the only other option would have been a "cold solder joint" on the Mother board -- which you've already replaced.
(( sorta narrows THAT problem to its trouble maker ,Eeh!?! ))
Hard Drives ... 500gig ... under $140.00 ... the actual question here is not Maxtor ( which I've had a lot of good luck with but not many others have liked ) or Seagate ( both hard drives have a long-long history of reliable long lasting lifelines ) but whether you are using SATA or IDE connections.
.. I prefer Maxtor EIDE for its super-fast read/write for editing ( also its a lot quicker than SATA in doing Auto-CAD )
.. Most other computer users prefer Seagate Serial-ATA hard drives; in fact, there's hardly a computer sold today that doesn't use SATA hard drives.
.. As far as 250 gig, 500 gig, 750 gig, when people were using Windows 98 a 200 gig hard drive was considered just about right for Video Editing. Today, I'd have to say that ±500gig should be plenty of room for creating an AMV