1) While booting up hit the F8 key untill you reach a boot up menu that allows you to get to choose command prompt only.

2) After reaching the C:\> prompt type in "scanreg /restore" (without the quotation marks; and with a space before the / slash) and use your up or down arrow keys to go up or down to the last date you remember the machine working properly and hit the enter key to choose that previous registry file to restore and then respond positively to what it asks afterwards to restore and reboot the system.