Kathmandu is extremely safe to most other cities. You can easily go out alone - just be careful with traffic as it can get crazy.

As a single women you'd have to take precautions similar to any city.

As the other poster says - it would be a true shame to only stay in your hotel at night. There are tons of fun places in the Thamel area you can visit at night (restaurants, shops and bars) and they are all perfectly safe. As to hotels - the Yak and Yeti is always good (and centrally located) and I personally like the Summit hotel in patan.

For additional info why not visit the thorn tree South Asia thread on the lonley planet website?