It is not difficult to do and 99.9% of the time it will work just fine, but you must be prepared for things to go pear shaped. I wouldn't attempt this before a full backup and I would want to have the original OS & manufacturer driver CD's (mobo, video, soundcard etc.) available just in case.
Commercial untilities like Partition Magic will do this. I use Partition Magic regularly and have never had a problem with data loss. There are similar freeware & shareware utilities available too.