When you bought it, there should have been an operating system CD supplied with it, which the retailer will have used to install Microsoft Windows.
If you can still find it, insert it in the CD drive (with the power on) then power off, then power on again. This will allow boot from CD and you may see some extra boot options including Windows Repair tool.
I have heard that the repair task can take quite a bit of time so possibly best to wait until the morning to try this.
DO NOT attempt the 'clean install' option. It will wipe all your user data and you will have to do all the Windows Service Pack and update downloads all over again.