Hehe, the same thing sorta happened to me the other day.
I put in my old network card again cause I just got my dsl back, and my computer turned itself on as soon as the network card started going into the slot...
Freaked me out... So, I totally disconnected the power and put it back in. Hasnt happened since.
AthlonXP1900+@2100+, Abit KR7A-Raid, Radeon 9700 Pro (core370/mem666), 256Megs PC2100, 2x 40gig WD7200, Soundblaster Audigy Platinum, KDS VS 19" CRT, Klipsch ProMedia 5.1s, Creative 12x DVD, TDK 24x CDRW, 500Watt PSU, Win2kSP3.