The two common areas are:

1. Motherboard capacitors.

Very easy to spot. Here's pictures -> http://www.pcstats.com/articleview.cfm?articleID=195

Fix? Replace the motherboard.

2. Power supply has degraded.

Along with the capacitor issue above, ELECTROLYTIC CAPACITORS age and lose capacity. A smaller power supply and heat accelerates the aging which is surprising to some that 1 year can be all that it takes to start seeing issues.

Since 450 Watt PSUs are in the 20 something range, there is no need to skimp here (a duron 800 won't need that size, but why skimp?) and is just a little cheaper than the motherboard.

It's likely to be one of these two items.

Less likely, but worth replacing is the CMOS BATTERY. It's a few dollars and if you want to make sure, just replace it.

Bob