MonkeyClicker

I have a dual boot system and when I am logged into Linux I can't access the data on my Windows partition.

Knoppix Linux, and some other distros, place icons on your desktop that represent the various hard drive partitions on your computer. Mounting is as simple (in most cases) as clicking on them. You may need to change the read/write status of the partition (be careful here). You can do that by right-clicking on the icon and choosing the option from the menu.

If you are using a distro that doesn't automatically add partition icons to your desktop, you just have to mount the partition yourself. This procedure works if your Windows partition is FAT16 or FAT32 First (as root) create a directory that you will use to access the partition. You can call it anything you like. Perhaps something like /cdrive to remind you that it is your Windows C: drive. Next you need to mount the drive with the command:
mount /dev/hda1 /cdrive -t vfat.
You should now have full read/write access to all the stuff on your Windows partition. You can mount the drive automatically every time Linux boots by adding the following line to your /etc/fstab file:
/dev/hda1 /cdrive vfat user 0 2
Make sure the /etc/fstab file ends with a cr or Linux will complain during bootup.

This should work for NTFS partitions too with most Linux distros. However, Micro$oft has never made the specifications for the NTFS filesystem public. People have had to reverse engineer the filesystem as best they could in order to create drivers to access NTFS partitions. These drivers are at best alpha grade software and not terribly reliable. So just to be safe, I wouldn't recommend writing to your NTFS partitions from Linux. Reading them is fine, but writing to them has a small probability of causing data corruption. In the above mount command, replace vfat with ntfs.