Intel imac hard drive free space errors comes up when I try to save microsoft wo

Hello,

I have an 2006 Intel iMac that I'm starting to have problems with. I have Microsoft Office 2004 for a Mac installed on it. I'm working on an important document that I keep loosing the work I've done on it because it won't let me save the document more than 3 times before I get an ERROR message that says I don't have enough hard drive space left on my computer to save it. It tells me to delete files or close out other programs and try it again. I have 72GB left on my 150GB hard drive. I don't understand why this message keeps coming up. Could someone PLEASE help me with this issues? Thanks, Jay

System Specs:
Capacity : 152 GB (163,208,757,248 Bytes)
Available : 79.8 GB (85,659,631,616 Bytes)
Used: 72.2GB (77,549,125,632 Bytes)
Number of Folders : 178,381
Number of Files : 820,875

Model Name: iMac
Model Identifier: iMac6,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.16 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 2.5 GB
Bus Speed: 667 MHz