You can try to boot from the VISTA DVD and do a repair or restore
or if you can boot into "safe mode" you can try this:
Symptoms include this message on boot:vista-icon
“configuring updates stage 3 of 3. 0% complete”
and then an automatic reboot and the same message over and over again in an endless loop.
Some people say to just let it sit while plugged in to the ‘net or to try a repair install but without any restore points you aren’t going to get anywhere.
For some odd reason they removed the Method 3 from this MS help article, here is a copy of it:
Rename the Pending.xml file, and then edit the registry.
To rename the Pending.xml file and to edit the registry, follow these steps.
Start Windows Vista and go to the System Recovery options:
1. Insert the Windows Vista installation disc in the disc drive, and then restart the computer.
2. When you are prompted to restart from the disc, press any key.
3. When you are prompted, configure the Language to install, Time and currency format, and Keyboard or input method options that you want, and then click Next.
4. On the Install Windows page, click Repair your computer.
5. On the System Recovery Options page, click the version of the Windows Vista operating system that you want to repair, and then click Next.
Use the System Recovery options to rename the Pending.xml file and edit the registry:
1. On the System Recovery Options page, click Command Prompt.
2. Type cd C:\windows\winsxs, and then press ENTER.
3. Type ren pending.xml pending.old, and then press ENTER.
4. Type regedit, and then press ENTER.
5. Select HKEY_LOCAL_MACHINE.
6. On the File menu, click Load Hive.
7. Locate the following folder: C:\windows\system32\config\components
8. When you are prompted for a name, type Offline_Components.
9. In Registry Editor, locate and then delete the following registry subkey:
Exit Registry Editor:
1. At the command prompt, type exit to exit Registry Editor.
2. Press ENTER.
3. Click Restart.