There are at least 5 different ways to approach such situation:
1. If I'm familiar with the area, and if know an alternative way, then I just ignore the GPS, and I
drive the way I want. Eventually the GPS program will catch up with you, because the new Microsoft Streets & Trips 2008 automatically calculates a new route whenever it notices you are a certain distance from the plotted route. You may press the F7 keyboard shortcut to mute Streets & Trips temporarily, to avoid listening to the instructions you don't wish to follow . (Press F7 again to un-mute.);
2. You could adjust the
zoom a bit to see the surrounding roads better. If you see what appears to be a possible alternative route, you could just start driving that way. Keep an eye on Streets and Trips as it starts suggesting you alternative routes;
3. You could add
stops to your route to force Streets & Trips to take you via a certain area, or avoid a certain area.
4. You could
click-and-drag a route segment to a different road, as depicted on the two screenshots above. (By the way, the two pictures are from the Help file in Streets & Trips. Do check the Help, there is a wealth of information there waiting for you.)
5. My most favorite tool to modify a route is the
Avoid Area feature. Click the "Select"

button, and then with your mouse draw a rectangle on the map, covering the area you wish to avoid. This is a very powerful feature, which allows for a convenient and very fast route modification. You may create avoid area rectangles of different shapes and sizes, and you may use one or several of them, to modify a route to your liking.