I was looking into similar tools a few moths ago and came across Issue Tracker on the ASP.Net website (in samples). There are currently 2 versions one in ASP.Net and one in VB.Net and both use either a SQL or Access back end. It is fully customisable and allows you to set up multiple projects on one database, assign users to projects, assign issues between users etc.
It has definately worked for us so may be worth taking a look at
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