Only Assassin's Creed 1 can be run but not on bad frame-rates. Beacuse VRAM needed should be 512mb and RAM should be 2GB for it. And for the processor, 2ghz is also less but can run it. better to upgrade your system.
To run Assassin's Creed 1, 2, BrotherHood and Revelations You need minimum these specs:
DualCore 2.4 (minimum) but 2.8ghz will be better.
2GB RAM
Minimum 512MB VRAM GPU (Direct X9.0c or above)