Fixing PC is still hard knowledge :(

I got the PC of my girlfriend last Saturday.
In the first try to fix it, I use a new hard disk and install a new OS, it still can't work.
Yesterday I spent NT 8700 for a new hard disk(WD 200G), a CPU(Celeron 2.6J), a Com CDROM(BenQ), a new RAM(DDR400 512MB), and a new ASUS mainboard.
After buying the above, I was confident that it could work smoothly.
I was still surprised in composing the elements into the old case.
The CPU's structure is 775, I have never seen the pins were on MB instead of CPU, so cool!!
After composing them completely, the LCD still showed me nothing but darkness.>_<
This morning I took the PC to be diagnosed where the problem is. It's marvelous that it could work in that place. But it still can't work in my room before 10 minutes. Maybe the bad part is the power supply or the case.

My uncle also ask me to help him buy a whole new PC.
The two new PCs made me very busy.
I hope I could finish them this week because I should concentrate on preparing TOFEL test at 20th, March.