You can either delete all restore points except the latest one, or all the restore points
To delete all restore points except the latest one, use the Disk Cleanup utility. Click Start, All Programs, Accessories, System Tools, and then Disk Cleanup. Click on the more options tab and then select Clean up in the System Restore dialog box.
To delete all the restore points on your computer, disable and re-enable system restore on the system. Click Start, Control Panel, and then the System icon. Click on the System Restore tab in the dialog box, select the Turn off System Restore check box, and click Apply. Clear the check box again to re-enable System Restore and then click OK.
You can reduce the number of restore points saved by decreasing the total amount of disk space available to System Restore. Note that less available disk space will decrease the relative number of restore points.
Any restore point that was created after getting the malware will be infected. In this case, your only option to remove the malware from system restore is to remove all restore points.