Togo, if i may say, you do not clean the bearing on a fan the ar basically a sealed unit, you can however clean the fins on the fan as you say with a paint brush, and a little servicol on the bearings, but a fans are so cheap, just replace them or one whatever you have, plenty on ebay, just check either three pin or four on connector. using a hoover is a good idea though. a lot of problems with old pc's is lack of memory, and low cpu ie old pentiums, least you want is an intel i5, lower than that an i3 but pointless spend a few quid more, i wont go into amd processors.