Just opening Photoshop causes it start reserving Scratch space. Video files can be huge, so Photoshop is going to be looking for a lot more scratch space than you might think. I have seen Photoshop temp files amount to in excess of two or three hundred Gb for instance, and not with particularl big image documents. Scratch space will be even more in demand if you son't have a lot of RAM.
What are your system specs? How many drives do you have, and how are they allocated to Scratch in Preferences > Performance? What are your other settings in Preferences > Performance?
Are you sure you don't have any drive permissions problems? We have seen a few of those here, and I 'think'a lot of them were with Mac systems. What OS are you using?
[EDIT] BTW Video can put a huge load on system resources. Especially if you are editing high def 1080 with something like H.264 (as used in most DSLRs) or AVCHD (as used on a lot of camcorders)