You should have tech support from MS or your manufacturer if upgrade came from them.
If not, upgrades can take a long time, rollback can take even longer. It depends on how badly you want to save your Vista installation for working through the in-place Upgrade issues after rollback. Fortunately you can do a clean install with Upgrade media -which is preferred anyway - unless you fully dread reinstalling programs and replacing files from backup (you do have one, I hope
If you need to bail out, hold the power button down for hard shutdown. Then restart and boot from DVD and do a clean install using this tutorial: Doing a Clean Install with a Upgrade Windows 7 Version
I suggest you format the drive using the Custom>Advanced tools for the cleanest possible install since you may well need to go that route anyway.