acer aspire M1640 desktop computer and 19' monitor .

Hey guys, This desktop has 2gb memory and 320 gb hard drive and comes with an additional acer 19' monitor which i purchased seperatly. Im not good with computers but something was changed last year as it wasnt running correctly, when i got i back from the shop the windows they had installed was updated and it does not recognise the windows code on the back of my computer. Im sure this is not a difficult fix, im assuming just needs the correct software putting back on. Before the computer started aksing for the code, it ran great. I had to get a laptop for university anyway so havent used the computer since. Also has a wireless mouse (not pictured) Any questions please text 07769220534