Repairing Windows will not make you lose any of YOUR documents. You will have to reinstall any windows updates.
It's also a good idea to backup anything you value - just in case.
When it locks up a log should be created. Hopefully this log has been created and could tell you the cause of your problem.
To view the log. Click start > Run > Type:
Now click ok. Then click on APPLICATION and also SYSTEM. Have a look in the list for ERROR (in red) Now have a read what it says. At the bottom of this link there maybe a link, click this and there maybe a fix waiting.