I've bought Mountain Lion but my iMac's HD isn't the correct format (Mac OS Extended(Journaled) instead of GUID partition what do I do?

Question

Can I just back up to Time machine, then reformat disk then re down load?

Answer 1

As mende1 stated, the two are different and must be both.
If you open Disk Utility and select the hard drive (the type, size, and serial number), not the volume (Macintosh HD), it will tell you at the bottom of the screen what the Partition Table is. It should be GUID. If not, then you will need to back it up, then repartition the drive. Also, copy the Mountain Lion installer onto another drive so you don't have to download again. It is in the Applications folder.
There are a few methods to get back where you are, including wipe, install old OS, restore from TM, install ML.
You might search the web for a tutorial on creating a boot/install disk using the Mountain Lion installer.
Another easy way is to use something like Carbon Copy Cloner or SuperDuper to clone your current OS onto another drive. Then, boot using the external you cloned. Use Disk Utility to re-partition the internal drive as noted below, then Clone your current OS back to the internal. Then, use the installer you tucked away to reinstall.
If running Snow Leopard, you need to boot using your install disk, then open Disk Utility.
If running Lion, you need to boot into the Recovery HD by holding down cmd-r on restart.
From there, open Disk Utility, select the hard drive, then the Partition tab.
Select 1 partition from the partition layout dropdown (or however you'd like, just don't use current).
Set the Name of the disk. Macintosh HD is the norm, but you can name it anything you want.
Set the Format to Mac OS Extended (Journaled)
Click the Options Button.
Set the Partition Table to GUID Partition Table.
Back on the partition tab, click Apply.