Stuff you can try:
1. Reinstall WinAMP, update to the latest version (5.111 as of writing).
2. Update the USF plugin to the latest beta available.
3. Make sure the .usflib file is in the same folder as all the .miniusf files.
4. Make sure you are not playing them from within a RAR/ZIP/etc archive.
Edit: For WhiteX + GCFreak: USF is a PSF-style music format for N64 games. There is a USF plugin for WinAMP that can play N64 music by basically emulating part of the N64. There are a ton of ripped and tagged sets at:
http://www.halleyscometsoftware.com/usf/
There are other WinAMP plugins available too with their own formats. Here are some off the top of my head, name of the plugin in brackets:
USF - Nintendo 64 (64th Note)
PSF - PSX (Highly Experimental)
PSF2 - PS2 (Highly Experimental)
GBS - Gameboy (Meridian GBS Player)
GSF - Gameboy Advance (Highly Advanced)
GYM - Megadrive (YMAMP/in_gym)
VGM - Master System (VGM input plugin)