– See if you can find some information about the problem in the Event Viewer.
– Use the msconfig tool, to disable programs that run at startup, and see if the machine starts successfully without them. If so, try re-enabling those programs one at a time, to try to identify what is the program causing the error.
– Make sure you have some good antivirus and antispyware programs, and make sure they are up to date. Restart in safe mode without networking, and perform a full scan.
– You can also try Installing or reinstalling the latest service pack for your operating system.
If all else fails back up your data, gather all your driver, OS and software disks together, then do a format/reinstall of you hard drive.