What are the complete hardware specs on that PC? That old computer might fall below Adobe's minimum hardware requirements.
If you mean subscribing to Premiere Pro CC, it likely won't run well on Vista, if at all. CS5 is the last version that has Vista support:
A bit outdated, but scroll down to Guidelines for Disk Usage:
and also read all of the articles here:
If your computer is not powerful enough, you may want to consider Premiere Elements (but you will need Windows 7 for that):