Think I read somewhere that certain keys are linked, hence you cannot press more than a certain number - only three, if you're unlucky - of keys at the same time:
http://www.dribin.org/dave/keyboard/one_html/
Oh... _r.u.s.s. basically said just that
What you could do is try to sit down with notepad and test out what your keyboard matrix might look like and what keys that result in a lock.
I poked a bit and found out that for example my QWETYIP and AWDCBMP-keycombos can work without locking, however, I think that varies from keyboard to keyboard. Also, I think Ctrl Alt & Shift can be used along with most combos.
- Arand