Number of issues here:
1. Your system does not meet minimum requirements.
2. Your CPU, disks, memory are not enough for anything other than DV.
3. What video camera was used with what codec on what OS with which version of PR. etc.
Some suggestions... and answer at least the 16 questions at the end of the Wiki article.
Knowing the full specs. for the source Clips will be very important. That an AV file can play elsewhere is zero insurance that they can be edited in an NLE, like PrPro. What are those full specs? If necessary, the great, free utility, G-SPOT can get you that info.
Those source Clips are likely the issue. Knowing all about them will help us help you.