Deleting the Restore Points will not be sufficient as more will be
recreated to fill the space you have allocated to system Restore.
To change the space you have allocated:
Open System Restore
Click System Restore Settings on the left side.
Change the slider as desired.
Or you could Turn Off System Restore.
Read this for more about System Restore:
http://www.microsoft.com/technet/community/newsgroups/faqsrwxp.mspx