*Moved to new post*

Since you had problems detecting it in GUIminer, I suggest you download the latest full CCC from ati.com. Here is 32 bit version, and here is 64bit version.
If you don't know what version you have, go to start menu, right click on computer and select properties. Under the system header, which has the Windows Experience Index, there should be a label called "System Type", which tells if it's 32 bit or 64 bit.
Install the correct one and restart your computer. OpenCL should be installed now if you didn't have it before.

If the computer is a dedicated miner, you can run a worker on GUIminer with the extra flags -w 256 -f 10. Bit if you, as I do, use it normally, run it using -w 256 -f 70, as that doesn't slow the GUI down nearly as much.