The EASIEST way to do this is to get hold of some decent backup software like acronis where you can IMAGE the existing OS partition to say an external HDD.
You can also create bootable recovery media so you can boot from say the CD and it will restore your image on to a (re-sized if required) partition.
However you have to be careful here since the OS partition will have to be set to ACTIVE (not PRIMARY) and on first boot it WILL BSOD so without a Windows recovery disk you are totally hosed up.
That's why I say Computers should ALWAYS BE SOLD WITH A SEPARATE OS INSTALL DISK even if Windows (and a lot of other bloatware is pre-installed on it --you've PAID for the Windows Licence.)
Otherwise you will have to re-format your disk and do a comlete fresh install of a Brand new windows system.
Without the Windows install disk you really are up against it if you need to do a system repair on boot -- messing about with partitions often requires this tool.