What headphones?

Looking for headphones for a computer in a room with much noise. Need something that will block out noise, and can be driven by a computer. Can you get something like this for $100 or so? What to look for and where? Thanks.
Have you considered the Sennheiser HDC 451-1 headphone with the noise-cancelling circuitry? The headphone is light, comfortable, and the noise-cancellation system works very well in environments that have a steady-state noise in the background. I use this headphone whenever I fly, and it virtually eliminates cabin noise. Audio Advisor sells this headphone system for $100. It might be just the ticket for you.
It's above your price, but the Etomatic ER-4s are fantastic. If you decide to go for more $$, I can highly recommend these. I've been using them for years for travelling by air. They reduce the outside noise by about 25 dB, and they sound great.
For under $100 I would go with the Sony MDR-V6. It is a closed type headphone that helps to block out noise and can be easily driven.
I have a pair of the ER4s too, and they are great! I would recommend the ER4ps though if you are running them from your computer. Or if you don't want to spend the 269 bucks, they have a new "reasonably priced model." Check our headroom.headphone.com.
Ditto plus the ETY's
Etytomic are ok. Especially good at isolating your ears from ambient noise. I returned my ER4's after a week -long audition. For $140 the ER6's are the "bargain" of the line, especially compared to the ER4's ($270). But NEITHER provide much LF compared to most other “mediocre” over-the-ear designs.