System Restore can often be damaged by malware, not just viruses, but the sort that anti-virus scanners fail to pick up.
I would suggest that you download and install two often recommended scanners, Malwarebytes' Anti-malware and SUPERAntiSpyware, and scan separately with those, (one at a time). They are both free, and the free version of MBAM does not set itself to load at startup, but SAS may do, so check the installation procedure carefully if you don't want it to do that.
How you do this is up to you. Malware, including viruses, can hide in the System Restore backup files so you may want to disable System Restore then re-enable it immediately before scanning, then after the scan create a brand new System Restore point. Disabling deletes all the backup files and that prevents any hidden infections from taking hold again.
However if there is a need for you to restore your system to a previous time and date, don't disable System Restore.
To disable SR, goto the Control Panel, open "System", then click the System Restore tab. Disable SR, click Apply, then re-enable it immediately and click Apply/OK.
Then carry out the scans with the manual scanners above. After that, create a new System Restore point.
I hope that helps.
Mark