You can download pc_drums_42.exe (and other stuff) from here
http://www.personalcopy.com/sfonts.htm and install it in some directory like Sounds. Then you need to download this
http://www.rgcaudio.com/downloads/freeware/sfz197.exe, and install into wherever it wants to be installed, and then you copy (or move) the file sfz197.dll into your REAPER/Plugins directory. Restart Reaper. Now you can add sfz as an FX to a track, and load into sfz the PCdrums. Then you have an FX that will play quite OK drums for you on MIDI notes. Insert a MIDI item on the track, and try out the piano roll. On some keys (a lot, actually) you will hear the drums and other sounds.
You can also check out this thread
http://forum.cockos.com/showthread.php?t=5888.
Note that the Reaper forum has a search facility that works very well.
HTH