Q: fixing a second hand iMac

Yes, you can take it to an Apple Store. You are going to need a bootable installer disc for it. If this is an old PPC version running 10.5.8, then you can look for a Leopard retail installer disc set on Amazon or eBay. You could possible purchase replacement discs from Apple Support: Contacting Apple for support and service. Use the number for your Country.

Apple will no longer work on the pre-2006 iMacs this forum covers in an Apple Sotre or at a central facility. See:

http://support.apple.com/kb/HT1752

If yours is a PowerPC iMac you will need to find an third-party Apple repair shop that still works on older computers.

So it comes to knowing your iMac family. For a start, what does "About this Mac" say about your processor type?

A lot of people have tried to clean up their mac only to have gotten themselves in worse trouble.  Somethimes it best to leave things alone.