According to the Epson website (as I read it), your printer can only be connected to other devices EITHER using a USB cable OR wirelessly. (i.e. you
can't have it configured to print from your computer via USB and from your iPad wirelessly):
http://www.epson.co.uk/gb/en/viewcon/corporatesite/products/mainunits/faq/12299/3421
So you will have to change the connection between your PC and the printer to a wireless one. NB: That doesn't require your PC to be wireless-enabled. As long as it's connected to your router (e.g. by an ethernet cable), that's fine. Your PC then sends the print message to the router, which then sends it wirelessly to the printer.
However it can't do that until the printer has 'found' your router and established a connection to it. (Once it's done that then, since your iPad can already 'see' your router, you can print from either device). But this is where things get tricky (because Epson has a totally pathetic website). The Setup Guide for your printer simply tells readers to follow the instructions in the Network Guide (as does the User Manual for your printer) but the Network Guide isn't even on the website. (Even if it was, I wouldn't trust it. The last time I located a Network Guide on that website it was for totally the wrong printer anyway!)
So you need to look at the documentation which came with the printer to see if you've got the Network Guide. (NB: It might not be on paper; it could be on the CD). You then need to use it to ensure that your printer can see your router. (NB: Remove the USB cable to your PC first). Then you need to change the printer settings on your PC, so that it uses a wireless connection instead of a USB one. Everything should then be fine (but it's hard typing with my fingers crossed!)
I suspect that you might find yourself in need of a bit of further help though, and (since I can't read the Network Guide for myself) I probably won't be able to provide it. So you might need to phone Epson UK Support on 0871 423 7766 (10p per min, 0900-1730, Mon-Fri). If you'd rather not use an 0871 number, try 01442 261144.
The key thing to remember though is that everything needs to go via your router. Your iPad is (it seems) already communicating with your router. Your desktop needs to lose the USB connection to the printer, and to talk to your router via its ethernet cable (or wirelessly if that's how it's set up). Finally your printer needs to 'talk' to the router as well, which is the part that the Newtwork Guide should tell you how to set up.