i will explain this in as simple terms as i can as i do not know your experience level with computers.
simple problem to fix and can be done many ways with little or no cost. of course there is always some risk if you do not know what you are getting into but it should be allright.
first of all you need to determine if there are any files on your computer that you want to keep, just take a look through the locations where you store your files such as my documents, music, pictures, etc.
if you do need to keep some files you can back them up a few different ways, the first would be to buy a external usb hard drive and put them on there, the other would be to create a seperate partition to temporarily store the files. to create a new partition download this cd image:
http://sourceforge.net/project/downloading.php?group_id=115843&filename=gparted-live-0.4.5-2.iso&a=78719092
and then this program to burn it:
http://download.imgburn.com/SetupImgBurn_2.4.4.0.exe
you need this program because burning the file to the cd will just burn a copy of the file and burning it as an image will create a bootable cd
after you are done reboot and go into your bios (details in your manual it is different for different computers) change the boot order to boot from the cd drive as top priority and then save settings, you will now boot gparted, press enter at all prompts while loading and then when gparted loads select your windows partition and select resize/move and shrink it so there is enough free space for your files and hit ok and then select the blank space and hit new and select ntfs from the partition type dropdown hit ok then apply. wait it may take awhile.
after this is done exit the program, click desktop and go to shutdown and reboot, remove cd so you boot windows. copy tour files to the new drive in my comuter and then shutdown, put cd in to boot gparted and now right click on the main partition and select format to then ntfs. hit apply and wait.
now reboot with the windows reinstallation disk your manufacturer provided and reinstall windows MAKE SHURE TO INSTALL ON EMPTY PARTITION! afterwards copy the files back to windows and use gparted to delete the temporary partition we made and resize the windows one
DONE!