We have sort of same kind problem with three PCs in our office. (2 WinXP, 1 W2K ... all Japanese Version)
I had tried with not only WEBRick, but also SCGI. The problems had not been resolved; this is not because of WEBrick. Also, I have installed Rails from scratch with two differenct version of Ruby besides the InstantRails, and they didn't work either.
Finaly, I have removed some applications from my PC, and then the problem has gone. The most suspicious@application was "secure application manager"--- but the name was in Japanese Characters---, I guess.
Before the action I took, I have checked file+directory permissions, because the server application seems not able to read/write something, or some event requests are hooked by the other process.
Why don't you check out dubious applications in your PC? Hopefully the problem would be resolved.