You don't say which virus it was but some such as blasterworm will drop into the restore point files thereby reactivating the virus if you restore the system ( you restore your system and the live virus ) it may be therefore that the purging has deleted the retore files as they contained the virus.
anyway assuming that the system is working as you wish why do you want the old system restore points anyway? If you like to keep them then go ahead and create as many as you want. Also they could go missing as windows has a size limit on the system restore files and works on getting rid of the oldest first.
Incidentally you don't say what OS you are using. but if you do you will get much better answers as this feture works differently in each version.