There can be different reasons, depending on the file type and the configuration of your computer.
The more RAM in your computer, the fewer error conditions you'll encounted with Registax. I use two computers for Registax. One has 2 gig RAM and the other has 256M RAM. In the larger and faster computer, I can load and process files up to about 1.5 gigabytes in size without a problem. The larger the file, of course, the longer it takes to process it. With the older, smaller, and slower computer, I can usually load files up to about 1 gigabyte in size if they are AVI files. But if they are separate bitmapped images, I run out of RAM in a hurry. About the largest number of 6-megapixel DSLR images I have successfully stacked on the smaller computer is 6.
If your AVI file is larger than 2 gigabytes, you will run into a Windows file-size limitation. I had that problem a lot when I used R3, but have not had it since switching to R4. I run XP Pro on the larger and faster computer, and Win2000 on the older computer.
There is a Registax Yahoo Group you should consider joining, if you have not already. Lots of helpful and experienced folks there, including Cor Berrevoets himself, from time to time.