You can delete the database, that's about as reset as you can get :)
Delete or rename: C:\Users\{you}\AppData\Local\Microsoft\Media Player
This also stands no chance of deleting the actual media files, unlike a slip/mistake with deleting from inside Media Player itself.
Win7 x64 / GA-MA78GPM-DS2H / Phenom 9350e / HD4550 512MB / 4GB OCZ 1066Mhz / WD10EADS 1TB / BDU-X10S Blu-Ray / FiOS ClearQAM / ATSC / TMT 3 + SimHD / emuCenter MAME / Bravia KDL-40WL140 / Roku N1000 for everything else MC can't do yet