You don't say which Windows you are using, but if it happens to be 98 then the simplest way......
Boot up and after the "Press Del to enter BIOS" option is on the screen, press F8 repeatedly until instead of loading Windows normally it will go to a 6 option DOS screen (including safe mode).
Pick "Command prompt only"
Then type scanreg /restore and enter.
Now you will have a list of the last five days when it booted satisfactorily. Choose the date just before things went pear-shaped and Enter.
Follow the prompts and it should be back to normal.