When something works one day, but not the next, it usually means that something has changed. This could be with the hardware, your Projects or perhaps the OS. It is less likely that it is with PrPro, itself.
Let's start with these QUESTIONS. Work down the list, and answer all that apply. There are also some troubleshooting tips in the link. Working though those might well help.
When you have most of the answers, also give complete details of your Projects that have failed/crashed. The Project Presets will be important, as will details of your Assets, especially the file formats and most importantly, the CODEC's used in these files. If you do not know that info, you can use G-Spot to find it out for most AV file types.
Somewhere between your computer and your Projects (especially the Assets), someone will likely be able to help you. Constant crashing is not normal, and obviously not good. That it worked well at one time, is a good sign.
I had the same symptoms like you, so I presume you are on a Windows OS like myself. The people who contribute have helped me solve other situations before. Like Hunt mentioned, it usually is because a change like a recent upgrade or update in a software that has taken place. Well, I read Hunt's and Harm's response to checking the latest events on your computer and I found that Windows updated my files ( even though I have it on manual) with something that affects or enhances Windows Media Center. I just have a nominal 8gb ram, duo-core processr, with three clean and non-partioned hard drives, and a 1 gb/256 bit graphic accelerator card computer which is set-up strictly for Adobe Production Suite. I removed all the games and goodies that came with the newly upgraded Vista 64-bit OS. Try manually downloading to update and "extract" the zip file of PrPro CS4 4.0.1. It's on this download site. It has worked for me. In the file it has a 3rd party user update. My computer wasn't recognizing my canon xlh1 with PrPro, but it could with the simple Windows Movie Maker. Therefore, I realized it may be a codec problem - just like everyone said. "These guys and gals are real good!"
Even though my preferences in PrPro is set to automatically update, it didn't automatically update that file. Now PrPro works like how it did when I first installed it. I hope this helps.
Ps. Also, if you are using your main hard drive's OS to capture and store, your computer is likely to crash when using PrPro.
Are you saying you have only one disk and it has only 20 GB free? That is inviting disaster to strike. For decent editing you need one disk for OS/programs, one for media and one for projects/renders/scratch.