HI !
reading your problem is kind of self-explainatory that how you must be feeling. dont worry, its only a computer and its only a OS, it can be worked around. so wel will sort it out 
first of all, as i suggest you. remove all the unwanted applications.
Now, uninstall your Anti-virus program.
now boot from the windows vista DVD and choose repair and select system restore. just restore your computer to an earlier time and i guess your problem will be solved.
if not, then just choose the other options.
boot into vista and type SFC /SCANNOW in run (if you dont have run in start menu either enable it or press CTRL+R)
insert your vista cd/dvd when prompted and let the system check its files by itself)
if the comman does not work then try this :
Open an elevated command prompt. To do this, click Start, click All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
cheers
shantanu