Which version of Windows are you using? Win2k? XP Pro? XP Home? If you never set a password for the Administrator account, that could be the reason why he was able to login through safe mode and delete your password for your other account. Boot into safe mode and set a password for that account.

Then reboot and in your computer bios, make a password be required to start up your computer... that way if he reboots, he would need the pass before anything even loads... quick and simple fix. Only way to bypass that is if he pops open the case and resets the cmos. Case locks? To be even more secure, use NTFS for your filesystem. If he gets pass your bios password and uses a regular dos/win98 boot disk that doesn't support NTFS, he won't be able to read any files. If you are even more paranoid, NTFS can also encrypt your files.

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-Kalvin