Help Desk Recommendation for 300 user company (internal users only)

I am looking to replace our ancient Help Desk software with something that better fits our needs at a resonable price.
Desires;
* Windows based
* Defineable user & technician access
* Email & Web interface for problem input
* Ability for users to update work orders
* auto/simple email to user as workorders are updated/completed.
* ability to reassign & prioritize requests
* Knowledgebase, accessible by users.

Are you using software you would recommend, or are there features you LOVE that are not listed?

thanks in advance,
Alan Brown