can't update, can't do a factory reset

I have a Nexus S running Android 2.3.4. For a while now it's been trying to prompt me to install newer versions. Every time I try, it restarts, displays the cute robot error screen and doesn't come back on until I take out the battery and restart it. Pressing power+volume+/- doesn't help. I also tried to do a factory reset. Same result. Before I got this phone (it was a gift), the shop did something to it "to enable Hebrew." Not sure what exactly but I suspect they dinkied around with system files and that's what is causing this. Anyone have suggestions for what to try next?