Pretty old TV you got there... *nudge*
RF Modulators, for the most part, suck. Because they output the signal over Coax on ch. RF3 or RF4, the best you can hope for is CATV quality - but don't count on it. If you must go the RF modulator route, then you should definately spend the extra $8.97 it costs to get a stereo RF Modulator versus a monoral RF modulator.
You'd get much better results if you go from the video out, via either the composite/RCA jack or the S-Video out of your computers video capture card/video card directly to an aux input of your TV.