Check for any open files that you can create in your tests. Depending on the size of your test suite, which can be too time-consuming and tedious. There it is possible to disable caching of the test process between test runs in the settings of TestDriven.Net. This is similar to what you see. From their documentation:
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: http://weblogs.asp.net/nunitaddin/archive/2008/12/03/testdriven-net-options-pane.aspx
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