How do I reinstall the system?

The simplest method is the 'system recovery' or 'restore to factory conditions' found in the user manual of your computer. It will bring the computer in the state it was in when you bought it.
That works either by booting from a special recovery partition by pressing some function key early in the boot process, or by booting from the recovery disks that came with the computer or that you were told to make yourself.

Alas, there is no standard for this. That's why documented it is in the documentation of the computer.

Kees

If you have nothing of importance on your hard drive i feel the easiest and most thorough way to do this to make sure it doesn't happen again is to insert your windows installation cd and reboot the pc. Upon rebooting if you see "press any key to boot from cd" than do so. If you dont see this and the pc trys to boot to the OS that is not working you then have to restart and go into the BIOS "documentation that came with your pc should tell you how to do this". Then you need to change the boot sequence from (1st device:HDD 2nd device: cd rom) TO (1st device: cd rom 2nd device: hdd) now restart and it should give you the option to boot from cd. Do this. It should now tell you that you need to reformat your disk do so. It will then tell you to partition the disk do so. It should then start to install a fresh copy of windows.

You can't mention reinstall with formatting or without formatting. If with formatting, Pls collect win7 OS cd and format the system drive and install win7. If without formatting you recover your OS.