you can have more than 1 wireless router on the network (well the second one wont be a router) so you can have it:
INTERNET ---- MODEM ---- WIRELESS ROUTER ---- dESKTOP PC connected via wires
---- Laptop downstairs wireless
to get the signal a little stronger you can run a cable from the router downstairs and chuck another wifi device on it so you can connect to that, perhaps use homeplug to do this.
Or perhaps use homeplug to just go from the router upstairs into the homeplug and then have the other homeplug in the workshop..
basically loads of options all depends how you feel about computers and money
