can I use iCloud on my computer? I want to back up my iMac and restore it since it is running so slow.

You need to be running Lion (10.7.x) to have iCloud run. Here are the system requirements for Lion:

• Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
• 2GB of memory (4GB strongly recommended)
• OS X v10.6.6 or later (v10.6.8 recommended)
• 7GB of available space
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

If your current iMac has 4GB of RAM then you can upgrade to Lion via the Mac App store and then use iCloud. However iCloud is not a backup system so please read up on what iCloud is designed for. To backup your system really all you need is an external HD and you can use Time Machine which is built into your system. Here is the Time Machine Tutorial to learn how to start.

no don't believe you can and if you could the 5GB you get for free would not be enough so you would have to pay super much more to get enough space

better / cheaper to get an external hd and timemachine backup to it and then reinstall and restore