You may find running Windows 64 bit and Sonar, with only 4GB of RAM, that your track and plug in count may be limited. This of course depends on exactly which plug ins you are running as some will require more RAM than others. Others here will be able to tell you which plugins and synths are the most RAM hungry. But Sonar X3d will run, just experiment and you will discover what works and what doesn't.
If you can give the computer a RAM upgrade you may find this will help more than anything else. Don't forget to tweak Windows, as this will help with XP.