As soon as you turn on your system, tap (and continue to tap) your delete key. Most mobo's bring you into your system bios this way. Some others are F8 or another key. Usually when you turn your system on, it will flash (rather quickly) "Press delete to enter bios".
When you tap your delete key during this process, it will bring you to a screen (usually the first screen will show the system clock and the boot devices attached to your system)
Across the top you will see specific menus. The one you will be looking for will be labeled boot sequence or something to that affect. Use your right arrow key to scroll to that option and either that screen will appear or you might have to hit enter (depending on the bios type and board manufacturer). What you want to do is to change the boot sequence from the first boot device (should be your hard drive) to your CDROM. Then you may have to hit escape to get back to the menu across the top. Select save and exit. This will automatically change the boot sequence to CDROM as the first device. Once the system reboots, it will now boot from the CD and the loading process should start. Follow the on screen options for fresh install. Once the process is complete and the system automatically reboots, don't forget to get back into the bios (same way as above) and change the boot sequence back to first priority to boot (hard drive), and then save and exit. The system will now boot into Windows from the hard drive. Remember to then insert your motherboard cd and install all motherboard drivers. Then ensure you get your virus and firewall up as soon as possible. You could wait until you download and install at least SP1, but don't wait too long because until you do, your system is at risk.
Let's just hope this takes care of the problem. I'm sure you know what to load next.
Good Luck!!