6 years old and low end it sounds like.
eMachines D620-5150 Specs - CNET
What's really causing it to chug is Vista. You don't have much processor power or ram and Vista is so poorly written it's unstable and is the shining example of BLOATWARE. It was a royal pain in the gazootenblatz to find drivers for hardware. It was so poorly written that a lot of aftermarket device makers didn't bother to write drivers for it. And the few brave souls that did had major support headaches.
Wipe the drive clean and install XP Home on it. XP performs surprisingly well in a low and slow system like yours.
That system uses the CPU to do all the video work and robs the main memory. That alone, for me anyway, would make me use it for a paper weight or a door prop.