This command scans your farm servers before starting an upgrade to ensure that some of the prerequisties are met for a successful upgrade.

This is only a guide and will not fix the problems itself.

Remember to work on a development environment and not production. Backup anything before you touch it  – even if its your development environment!

The pre-upgrade scanning and reporting operation is implemented as Stsadm –o preupgradecheck , and can be run with or without parameters. For more information, see Preupgradecheck: Stsadm operation (Windows SharePoint Services).

You can use this Stsadm operation to scan farm servers before starting an upgrade to ensure that some upgrade prerequisites are met and to detect known issues that can prevent the upgrade from completing successfully. The results of the scan enable you to address any issues that are identified.

The upgrade checker does not do the following:

• Supersede the Microsoft Best Practices Analyzer for Windows SharePoint Services 3.0 and the 2007 Microsoft Office system.
• Automatically fix issues that are identified.

For more information type stsadm -help preupgradecheck

More information can be found here:

http://technet.microsoft.com/en-us/library/dd793607.aspx