I've just had my first stab at making a video. I found that if I set LFS/Fraps to 30fps as the first post suggests my 6 year old PC could not cope. My solution was to set both LFS max frame rate (see 1.png) and Fraps (see 2.png) to 14fps (for some reason LFS would not let me pick 15fps) and then capture the replay at half speed as suggested in the original post. I could do this at 640x480 and maintain the frame rate.
Then in VirtualDub I found it necessary to double the frame rate in two places in Video->Frame Rate... The "Change to" box and "Convert to fps" box (see 3.png). Here I enter 28fps :o. In the Audio->Filters... I used "new rate @ 88200" and "resample @ 44100" filters to get the sound back to what it should be (see 4.png).