I suspect you’ve ‘corrupted’ something in the Vista start up routine. Here are some things to try but you will need a Vista DVD.

If you don’t have a Vista DVD you can download/create a Recovery Disk here http://neosmart.net/blog/2008/windows-vista-recovery-disc-download/ From the downloaded image on your disk, create a CD in ISO format (a DVD is not necessary as it’s only 120MB in size).

It works in the same way as the Vista DVD, i.e. boot from the disk, choose ‘Press any key to boot from CD/DVD’ and Vista will start to load the Windows files. Select your language, time/currency and keyboard then click ‘Repair’ in the bottom left corner. Within the repair option there are five choices: Startup Repair, System Restore, Backup Restore, Memory Test and Command Prompt.

1. Run System Restore from a date before you started having problems. http://www.vistax64.com/tutorials/76905-system-restore-how.html?ltr=S
2. Run Chkdsk from the DVD http://www.vistax64.com/tutorials/67612-checkdisk.html
3. Run the System File Checker. This can repair many corrupted system files. It will also identify in the log file any corrupted files it could not repair http://www.vistax64.com/tutorials/66978-system-files.html
4. Copy any system files from your Vista DVD that SFC (number 3 above) identified as corrupted and could not repair http://www.vistax64.com/tutorials/86959-access-vista-install-dvd-files.html
5. Run a Vista startup repair. This can repair a lot of problems http://www.vistax64.com/tutorials/91467-startup-repair.html
6. Do a Vista upgrade repair reinstall. This gives you a new OS without changing settings, files, folders, other programs and the like http://www.vistax64.com/tutorials/88236-repair-install-vista.html

Here is your 21A error http://msdn.microsoft.com/en-gb/library/ms797159.aspx