It is possible to gain access to your computer without the password if you have the Windows CD and serial number to hand. You need to use the Repair facility. The downside is that using this facility will replace system files on your computer with the files on the CD. This means that if the CD is old then you will need to update your OS after the repair. Of course you would have to do this after a reformat anyway, but the repair should not affect any of your installed programs or your settings. Another possibility is to use a password recovery service. There are other methods that work, including using a specially prepared Linux boot disk with NTFS drivers and software that can read the Windows registry.
This service, from a reliable UK site, is free if you wait 3 days to receive the password. You need access to another computer with Internet access. Clearly you have this, else you could not have posted here.
If you decide on the Repair route, your best option is to use a CD with SP3 integrated (slipstreamed) if you are able to borrow or create one. This will save all the updating afterwards. If you are unsure about carrying out a repair, read
this page for full details.
You need to be alert for the correct moment to interrupt the installation of files during the repair operation.
After the computer reboots automatically (which happens after files have been copied), watch the screen carefully. "Installing Windows" will start. Soon the progress bar for Installing Devices will appear. When you see this progress bar, press
SHIFT + F10. A command console will open. At the prompt, type
NUSRMGR.CPL and press the Enter key. This will give you access to User Accounts. Change the password to something you will remember, or make it blank. Then close the window and exit the command console. It is important that you now continue with the repair, else the new password will not be set.