Hi Chris,
Unless you know what you're doing (which I guess you don't, otherwise you wouldn't be asking!), use the rescue CD that SEP suggestes - I've used it a few times.
Another alternative is use a Mandriva installation CD (DVD).
Mandriva includes an option for "Installing using space on a Windows Partition".
[Note: other Linuxes may have this facility, but I've not found them/used them]
This will allow you to resize using a nice, simple graphical tool.
I've done this a couple of times too:
(After defragging your HD *AND* backing up any important data!)
1. Boot from Mandriva install CD
2. Go through install process up to point where you have the resize partition option. From memory, it is the option that says something on the lines of: Install onto freesapce of Windows partition.
3. Resize partition as appropriate
4. Bomb out of the installation process (reboot).
5. Disk is now repartitioned.
If you are wanting to install linux, I would strongly reccommend that you create a large FAT32 partition as well as your linux partitions. This way, you can read/write the FAT32 partition from *either* Windows or Linux - use it to store your data, and you can access it regardless of the OS you boot into...
Good luck (and don't forget to assign points! ;-)
Regards,
Andy Bruce
I Love it when a plan comes together!