Possible HD problem: Can't install sofware, each time Disk Utility boots up

06-16  Source: Network gathering  Views:150 

Each time i click on a Dmg file to try and install some software it goes right to the Disk Utility program. The best I can do is click "Verify Disk" which works out ok but still can't install the software.
My HD is almost full using 106 of 111 available gigs leaving me only about 5 gig free space. Is there some disk utility I could run to figure it out? Perhaps some program like Dfrag for the mac. I tried to download & install iDfrag but again, I was stuck at the Disk Utility that boots up after downloading it and clicking to install.
I ended up buying a 500 GB Apple HD that had been in a macbook pro previously. The guy I bought it from for $105 told me it has SnowL mac OS already on it.
He also told me that I could easily clone my current drive on this drive by using Carbon Copy on this Bomb Bich site: http://www.bombich.com/
I downloaded the app but haven't ran it or tried to install yet because he told me I need to get an "Enclosure" first from a local store that will run me $17.
I'm about the buy that tomorrow and then go to install it.
He also said I could first try just Copying my couple of programs like Word & Excel over to the new drive and see if it'll work on the new drive. If it does, then I could just Copy the rest of my datafiles over because I don't have much actual software I need on my current drive.
My current OS is OS X ver 10.5.8. but he has the newest Snow Leopard on thenew 500 GB drive I just bought.
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