You can use a Ethernet cable but that is a bit complicated to make the set up.
Those are th things that come to my mind.
the Ethernet cable if the firt solution but is the most messy. To find a zip drive or an external CD burner for the PC and burn the files from it would be the best solution.
if both computers are hooked up thru a cable modem send it over the internet or through aim or something. If the files are small enough send an e-mail to yourself.
Are they hooked up to a hub or router if so it isnt that painful to get the mac talking to the pc.... if you just have an ethernet cable it wont work to plug it into both machines, this requires a crossover cable, the wiring is different in these.
well you would have to buy a crossover cable, then you would make a new profile on the windows machine (log and pass) then go to the properties of your "c" drive and click the sharing tab... then click new share or group or something like that. Add that new user to the share group and give them all permissions (read write etc) Now are you using OS X or 9??
Long answer: Hook up both computers through an ethernet cable with either a cross-over cable (Mac may be able to auto-sense so you might be able to use a straight through) or use a hub or switch (ignore the internet for now).
On the iMac, in System Preference, in Sharing turn on Windows File Sharing. From your PC add a new network place and type in \\your.ip.address.here\username and then your password. Then copy all your files from the PC to your iMac. Time for cake.