PC won't boot after Ubuntu/XP installation

Tonight I've formated both HDDs I have, one 40GB and other 160GB. First 40GB drive is primary master, other is slave. What I did is I've installed XP on 40GB drive, and then Ubuntu on 160GB, booted with grub normally, choosed Ubuntu in grub, so Ubuntu started booting up, forced a check and tried to restart - but the screen remained black and it was just restarting and restarting my PC. I've turned it off, then on, PC starts booting up, does all the checks, writes that it recognized all my hardware and then stops, just like that, the picture on the screen is with the last message that my UBS memory stick is recognized - and that's it. I've tried to plug out USB, open my PC and disconnect one HDD, then other HDD, tried to switch boot priority in BIOS, boot with Ubuntu CD, nothing helps, it just says my HDDs and optical drive are ok and recognizes and that's it. It doesn't want to start grub or boot or anything... The weirdest thing is that it doesn't give me any error, just stops... Any help? Thank you all in advanced!