You can actually fill a mechanical drive almost all the way up. You need to keep 20-30% of an SSD free so TRIM works correctly. Having an 80GB hard drive that is likely 10 years old or more is hurting you more than the drive being full. You can get a modern 1TB drive for around $60. It will be several times faster than any 80GB drive. You may actually have an ATA drive though and they don't even make them anymore. SATA has been the standard almost 10 years. If it has a wide ribbon cable it's ATA. A small cable means SATA.
is it possible to take out a laptop hard drive and put it in a desktop?
Yes, In fact I have a 2.5" SATA Hybrid drive as the Master HDD in my desktop right now. Usually you need to buy mounting kit, but with some creativity I found my own way to mount it. But the SATA and Power connections are the same. Not sure for older PATA drives though.
It combines a 500GB mechanical hard drive with some SSD memory. I looked at all the reviews and benchmarks. It's truly amazing. It remembers what programs you use most frequently (such as startup) and assigns those to the SSD portion.