You can a wireless PCI card for a PC pretty cheap these days. $20-40 at a store and most likely cheaper online.
Otherwise, assuming that your iMac is connecting through airport, head over to your system preferences and over to Sharing. Select Internet Sharing on the left (if you cannot select it, the open window should be telling you what you need to do in order to). Basically the goal is share FROM airport and then "to computers using" ETHERNET. This will take in the airport signal and then send it out over ethernet to whatever computer is then connected (remember to then check the box for Internet Sharing after you select which connection to use)
In Windows, it should see it as a normal LAN connection under the Internet Connections (or wherever it is in Windows that shows available net options such as dial-up, LAN, wifi or whatever).
Then, it's just a matter of not letting the iMac go to sleep if you want to use the net on the PC.
Oh, also a PC is sometimes finnicky about this, so you might want to try restarting the PC after you set it up to use the LAN. Or if the PC is old it might require a CROSSOVER ethernet cable as opposed to a PATCH cable. The Mac is smart enough to handle either but I'm not sure about the PC.
By all means this does and should work.
