Hello. Yes, you could replace the hard drive, but that is pretty extreme, and you would need to reinstall everything. The better way is to simply figure out what the password is. Here is some information for you.

First of all, make sure you really forgot the password. Remember the username and password are both case sensitive so make sure you use the correct capitalisation, Also make sure your number lock or caps lock are not turned on.

If you can't get in this way, you can follow the next instructions. This is a free service, if you have time to wait, but you can pay for a speedy service, This is a service to reveal user names and recover passwords for Windows NT, 2000, XP, and Vista. As long you have physical access to the computer, your passwords can be recovered.
The first thing you need to do is download the following file:

http://www.loginrecovery.com/image.exe

Then follow these instructions:

http://www.loginrecovery.com/instructions.html

You can also recover your passwords by using a utility designed for that purpose. Here are links to tools that can be downloaded and will find the password instantly. The downside, of course, is you have to pay for it.

http://www.prosoft3d.com/passwordrecovery/

http://www.password-changer.com/

There are other tools available that are free, but they require a little more sophistication. Here is a link that will provide information about 4 of these:

http://pcsupport.about.com/od/toolsofthetrade/tp/passrecovery.htm

Hopefully at least one of these should get you in again.