I'm in the same boat. I have an old PC as weel that does ok with video editing. A couple of things may be the problem with your caputering.
One thing may be the rpm on your hard drive basically a slower rpm will make running things on your pc slower especially video files.
Your PC hard drive may be full. since you mentioned you only have a 80GB HDD that's pretty small for video editing. I have a 320GB HDD and I have to clean it our every week just about to make room for videos.
Try defragmaenting your PC and deleting old files and progrmas you may not use.
You might just be better off getting a new PC. With the new AVCHD cameras and things like that you need a minimum amount of GHZ on your proccesor even to capture that type of video with certian programs. Hope this helps.
I am also looking for a new PC right now check out my thread at the two I'm looking at.
I have a 5 year old emachine with a Intel Celeron D processor and a 80GB harddrive. The CPU runs at max during video capture and burning. Is this processor way too slow?? Is there software that can help?? I'm looking too upgrade to a new machnine but thought I'd throw it out there first.