I built a new computer and it keeps freezing. why?

every time it gets started up the computer freezes shortly after. sometimes it doesn’t even load the icons in the taskbar.
the only problems with it i could think of would be the ram timings or the 145w PSU i have been meaning to replace. the specs are:

-QDI Platinix 2D motherboard
-D-Link Air Plus G DWL-G510 pci wireless card
-intel celeron 2.4GHz cpu
-Powercolor ATI radeon HD 2400 pro pci graphics card (256mb)
-*1 stick of kingston KVR256mb ram
-*1 stick of hynix 333mhz 256mb ram
-Quantum fireball lct 20gb IDE hard drive
-*145w PSU
-Windows XP home edition

*denotes the suspected problems.

so what might be wrong with it? i had changed out the cpu but it had been freezing before…