We have been using Redmine <
http://www.redmine.org/> for almost two years now. It's a really slick system and will do AD/LDAP authentication. It's free and will run nicely on a virtual machine.
You can also check out OSTicket <
http://osticket.com/>. Also free.
We used the Ticketsmith module from Dotproject (free) for about 5 years, but I wouldn't recommend it -- we hacked it up quite a bit to make it do what we wanted.
If you're willing to pay for something, IssueBurner looks pretty decent. <
http://issueburner.com/b/pricing>.