It's a relatively old computer; Slow hard drive, slow processor, outdated videochip and slow RAM.
Flash is probably never going to run very well again unless you turn down the graphics all the way. This can be achieved by right-clicking in the Flash application and changing the quality of the video.
If your entire Windows installation is slow you might want to consider a re-installation if it hasn't been done in a few years. Could net you a performance gain. Make back-ups before you do so, though.