You do need a fast comp to do it : / It uses a lot of the processor and video cards power, to render the image, then take 5-60 pictures of it per second, and store them somewhere. Since you only have 128mb of ram I imagine its trying to store them in the pagefile on the hard drive.... which would go really really slow.
Using an Athlon 1.73, Geforce 2 Pro, and 512mb DDR I can capture 320*240 at 60fps while running the emu at full speed.
To speed it up a bit, turn off the sound and any extra options (AA, 32 bit, ect), set the window res to 320*240, and just capture the small part of it you need at say... 10fps.