BleepingComputer.com: Windows Vista Upgrade Did Not Finish Installation

hi. i'm currently using Windows XP pro sp2. i decided to change it with windows vista so that i can hav my pc a new look. my system is 2.66ghz intel celeron. i have 512mb of ram and nvidia geforce FX5500 256mb HDD 80gb and 20gb. i used the 20gb for XP media center edition.

im installing vista in place because i do not have a dvd burner. so i began with the setup and at first it went well. and after 4 long hours, it finally reached the "completing installation"stage. since it was too long again to wait i decided to leave the computer. and when i returned the screen showed choices on how to start vista. its the one with safe mode, safe mode with networking last known good configuration and start windows normally. from then on i feel that there is something wrong because that screen should only show up when windows has not successfully booted up. so i chose start windows normally.i saw the green progress bar but after that, the screen goes blank and the reboots. and when it reboots, shows again that windows has not started up correctly. and then i chose safe mode. it booted up but said that installation cannot be completed in safe mode and needs to reboot.i still haven't rolled back the installation.

what will i do?tnx in advance!