Please don't tell us you tried an fdisk to shrink your OS partition?
Striek is right, you can possibly recover the data, its just that you'll have to purchase a recovery tool.

General rule of thumb, is that most partitions (unless otherwise specified) are static, meaning that you cannot move/resize the partition. The exception to this rule in Windows 2000/XP is that in Computer management, you can choose to set your drive up as a dynamic drive, and then can resize the partition, but this must be done BEFORE any other OS's are installed on your disk.

If you tend to resize partitions a lot, you might want to grab a copy of Partition Magic, this comes in handy, and is really nice if you want to take the complications and headaches out of partitioning it using standard OS utilities.

Good luck!