Capturing at 640x480 pixels requires a high-end PC configuration. Here are a few things you can try to improve capturing:

1. Turn off the preview during capture from the Tools menu of the Capture tool
2. Decrease the capture resolution from Video Properties or Camera Properties
3. Instead of capturing to uncompressed AVI, try capturing to the WMV format

Optional:
1. When capturing to WMV, change the WMV profile to one with a lower bit-rate
2. Defragment your hard-drive.