iMac Mid 2007 Very Slow

[Oliver G]

I own a iMac Mid 2007 (2.4Ghz, 320GB, 2GB) and in the last few weeks it has been running very slowly. It takes about 3 to 4 mins to boot and only lasts about an 1 hour before Finder freezes and requires a reboot.
When I try and reboot it gets stuck on the grey apple screen and never loads. But when I try about an hour later it boots up in 3 to 4 mins.
I have Repaired the Hard Drive and Permissions but this does nothing. I have only used about 45% of the Hard Drive. Is there anything I can do to get it back as a normal working Desktop Mac?

[wessew]

In situations like this I recommend that you try Diskwarrior. It is a directory optimization utility which analyzes and repairs the directory structure. I have found it to be safe and reliable. You will need to restart using the disk or a bootable backup with Diskwarrior installed. In over ten years of use it has returned my Mac to like new performance a number of times without a single issue. Highly recommended.