BttF and Puzzle Agent are two different games. Your PC being able to run BttF does not automatically guarantee that it can run Puzzle Agent. Again, you only have about 1 gig of RAM and 128MB of VRAM, both of which are about half of our min. specs.
Given that, it's not surprising that Puzzle Agent performs poorly on it (I'm surprised you can run any of the BttF episodes at all). If you want Puzzle Agent to perform better, you're going to have to upgrade your system to at least these min. specs:
OS: XP / Vista / Windows 7
Processor: 1.8 GHz Pentium 4 or equivalent
Memory: 2 GB RAM
Video Card: ATI or NVidia card w/ 256 MB RAM
Direct X: Direct X 9.0c
Audio card required
If upgrading isn't viable for you, then I advise either playing the game on some other machine or contacting customer support for a refund.