PC starts automatically but doesnt boot

Hello,
I recently had some power failures at my place and hence my hard disk got faulty so i got a new one. But now with the new hard-disk , when i switch my UPS on my pc starts up automatically BUT doesnt boot or anything No bios screen , all the processor fan / GPU fan and stuff runs , my harddisk runs too but my moniter remains dead/ not boot. i had all the bios screen like 2 days ago before i got a new one and all show up only when i tried to install windows xp , it would say that no storage device exist (bad hard disk , hence i got a new one ).Now after i got the new harddisk, my PC starts automatically ( not runing any normal functions , no boot) and doesn't load up bios or start screen or stuff .it simply just wont boot but all the fans of my processor and other hardware is all powered up and runing . please help me and sorry for bad english.
What i really dont understand is :
1. Whys is the computer starting itself up when i switch my UPS on but doesn't boot
2. If there's a problem with mobo or processor , How can a problem occur @ the time when im not using the Pc/ not runing it lol ?
3. Can there be problem with RAM or Power supply unit ?
God i hope my mobo/ ram /processor are okay else i think im going to cry

PS:
mobo- DG33FB core 2 duo , its like 6 months old
RAM - DDr2 1+2 Gb
160 GB S-ATA harddisk
Geforce 7200 LE