If you mean that you can not log into the desktop environment--then it might be necessary to use a Recovery Disk to start the computer but if you are able access the desktop then the following may solve the problem. See quote and link below:

"The file AtBroker.exe is a legitimate component of Windows Vista. The size of the file 28160 with the MD5 signature of 89AE93A81985A65BFA55071DAA8C7CD0. Windows Vista relates to a line of operating systems.....

If Windows is unable to start Windows Vista or Windows Vista crashes or freezes, you may have a registry error related with AtBroker.exe. Such errors can heavily influence the performance of your PC. We strongly recommend you to cleanup registry before making any changes to it...

Although AtBroker.exe is a legitimate Windows Vista file, some versions of Windows Vista have vulnerabilities. These security holes allow hackers to attack your computer and gain access to your financial and private data.

"

http://www.pc1news.com/files/583234-atbroker-exe.h...

If you have access to your Anti Virus Program. Try running it as a precaution. The trouble could be a virus or malware hacking through the above mentioned vulnerability. If none of this information is useful refer to the Source(s) List.

Good Luck.