In your /etc/fstab file (open as root) you should see a line something like this:
/dev/hda1 /media/hda1 vfat defaults 0 0
The vfat (it could also be ntfs for Windows 2000/XP) part is the filesystem type (windows type). This is how you know which line to edit. Add to the part after that (the options) user, so it will look like this:
/dev/hda1 /media/hda1 vfat defaults,user 0 0
Your file probably won't look exactly like that, it depends on what partition your Windows is and what the options are set to. The important thing is the user option.