I suffered a damaged partition table on my hard disk after doing a defragment on Windows 98, even though the defrag reported no problems.
The symptoms were basically that the drive letters (D:, E:, F:) had disappeared, and the PC thought it only had a C: drive.
I managed to retrieve files from a disk with a damaged partition table using the free software PCInspector - http://www.pcinspector.de/file_recovery/uk/welcome.htm
- fantastic! Took about 4 hours to search a 40gb drive for possible lost partitions, but got my data back fine. You'll need a spare hard drive (or a spare partition if you're feeling brave) to recover the files onto.
System Rescue Cd
is a free bootable CD which includes tools for partitioning disks and recovering data - http://www.sysresccd.org
It provides free functionality similar to Partition Magic
& Drive Image
You can also use FDISK if you're partitioning a new drive (or unpartitioned space on a drive) - it's very simple, but you'll need to be confident with command line software.