Computer doest boot but powers up

Hi, I have an hp pavilion 8670 c. 600mhz PIII 64MB ram,standard asus mobo,regular sound card, 32mb ati rage fury pro vid card, nic,56kmodem, burner and dvdrom,30gb and 40gb hd,no floppy. SOMETIMES when i start the computer it powers up but nothing happens... i hear it access the Hard drive for a short time then nothing happens. I have to turn the computer off and on several times until it boots. Sometimes it doesnt boot 30 times in a row... sometimes on the second try. Normally when it boots there is a beep but i dont get anything... just sits there with no feed to the monitor or anything. When it does boot it works flawlessly.... no crashes/freezes/bluescreens or anything... i have no clue what the problem could be... any help would be appreciated. thanks.

i just upgraded from a PIII 1gHz, 192mb RAM, TnT2M64, 30gig hdd, TO a PIII 1gHz, 448mb RAM, gf4Ti4200 128mb RAM, 30gig hdd. in other words, same processer, same hard drive, same floppy, same cd-rw, added RAM, new case, new mobo, new vid card, new sound card. i used the hard drive just b4 i took it out, worked fine. after i built the new comp, all it would do is turn on wouldn't boot, the fans work fine(PIII and vid) and the cd-rw and hdd fire up, everything is connected just fine, but nothing happens!i have tried 2 different grfx cards and 3 different moniters. my friend is going to look at itin a few days so i hope i don't need any help, but until he looks at it, any suggestions other than get a hdd.