The 300 Watt PSU is a likely suspect. It's aged and may not handle a power demand from the system.

Many will think that's not it. I will not debate such since a new 450 Watt PSU that removes this from the issue list is under 20 bucks around here.

Also, no mention of latest BIOS, default safe bios settings, memory tests with http://www.memtest86.com and if you installed the motherboard drivers. Lockups are easy to produce on demand in windows without these items.

Best of luck,

Bob