Variance is a measure of your computer's response time. If iti s bouncing all over the place, it simply means the CPU is busy handling things other than Aces High.
Could be spyware/malware, DOS attacks (particularly if you have your firewall report every port scan/hit, which is a bad configuration), or excessive swapping. Those would be the major candidates, but there are many others.
With no other programs running, open the Windows Task Manager and under the "Processes" tab watch the CPU (bottom center) for a minute. In a normal system, it will switch from 0 to 2% at regular intervals. If it is going higher than 2%, the some other software is using up CPU time which should not be running.