System 7: How to unlock screensaver?
There are a couple options.

lhf is right -- holding down shift will disable all extensions on startup in System 7.x. If you are running OS 7.5, holding the shift key will bring up the extensions manager, allowing you to choose the extension you want to shut off.

If that does not work, you can boot from a disk as if you were going to install software. You can download Mac OS 7.5.3 from Apple. You technically only need the first disk to boot. Once you create the disk from the first image, boot from it by holding down command and option. 

That should boot into a very minimal version of System 7.5 and bypass the screensaver.