wait... why would you want to password protect it upon startup of your computer if you really just dont want them to restart the computer? I mean, if they restart it, and then they cant turn it back on, you're still going to be losing your IMs, etc..
why not just put a password on your screen saver? then when your computer isn't being used for X minutes, your computer will be locked.. that combined with the standard Windows user password upon startup of WinXP is more than capable of keeping 99.9% of people off your computer. Also, if you setup a password for your Administrator's account, then they would not even be able to reboot into safe mode.. your computer would be locked down... and you wouldn't lose your IMs since your computer would still be running... and they would have no reason to flip your power switch..