Some computers don't have a windows install disk, or a restore disk and you need to produce restore disks yourself (based on the contents of a hidden directory). You might want to do this. Then if say your onboard NIC goes out and you need to send it in, you would restore the system before sending it in. There shouldn't be any problem then regardless of their policy.
If this is a Vista machine, I would recommend defragging and then resizing the NTFS partition using Vista's tools before installing Linux on the hard drive. When I did this, Vista was very greedy how much hard disk space it wanted for itself (only sparing about 30GB of a 120GB drive. I think my mistake was not defragging it first.
After installing Linux I used gparted to free up more space and had to repair both the Linux and NTFS partitions. I think I should have run Vista's defrag tool after resizing and retry resizing. My guess is that the MFT or other items were located at the middle of the drive preventing further reductions in size. Defragging again may have moved items toward the new middle of the partition allowing further resizing.
If you do install linux on the same disk with Windows, I would recommend manually configuring the partitions. This can prevent surprises such as accidently installing on the entire drive.
There are howto's on using the Windows bootloader to chainload linux. Vista doesn't have a C:/BOOT.INI file, but if you create one, it will use it. So you can follow the same instructions for NT or XP. If you google for "NT Linux dual boot" you should find many webpages with full instructions. This method will leave the original MBR intact while allowing you install Linux on another partition on the main hard drive.