Well, the quick-and-dirty way to do it is to scale your x component by cos(t) and your y component by sin(t). You might be happier with a matrix-based solution in the long run, though.
Edit: The math above is, of course, totally bogus. I plead momentary imbecility. johnnywang has the correct solution. D'oh!
What kind of program are you writing, btw?
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