Yeah, I know what you mean. It can be a real pain in the arse. However the problem is quickly solved by:
i) Switching on your modem
ii) Going to control panel>modems
iii)Atempt to add a new modem and let win2k search for a new one. It will not find one but will magically rediscover yours on COM1, COM2 or whatever, saving you a reboot.
Hope that helped.
Pete