You have 2 choices to make.
1, Upgrade the hardware
2, Use an older version of the software.
This stuff doens't run well on low end systems.
These are the recomended requirements
Operating System: Microsoft® Windows® XP Professional SP2 or later (recommended), Microsoft® Windows® XP Home SP2 or later, Microsoft® Windows® XP Tablet PC SP2 or later, or Microsoft® Windows® 2000 SP4 or later
Web Browser: Microsoft® Internet Explorer 6.0 SP1 or later
Processor: Pentium® 4 1.4 GHz or AMD K7 1.4 Ghz (minimum), or Pentium® 4 3.4 GHz (recommended)
RAM: 1 GB (minimum), or 2 GB (recommended)
Video: 1024 x 768 with True Color (minimum)
Hard Drive: 650 MB free hard disk space and 75 MB swap space, or 1.3 GB for full installation
When Auto cad tells you something I've learned to listen to them. This software is not built to run on low end systems. It's not going to work well. You can get away with it, but it's not going to be fun, or easy.
Sorry, hope you can find a solution.