The temperatures you have quoted are within range I would say.
Have you got any diagnostic software to give you the hard drive temperatures ?
I am now thinking down the PSU route - 350watt sounds a little under rated. (Hurriedly tries to find the advice Richard gave me re. PSUs ages ago which was spot on! )
Doesn't explain the sudden appearance of the problem though.
Interesting one.
When you said it died during the disk scan - what did the PC do ?
Blue screen ? Shut itself down ? Froze ?