My personal view of the OP Pro anti-spyware plug-in, a waste of time and processing effort, it is based on the AdAware program so if you have that or Spybot Search & Destroy or any other anti-spyware solution.
If you leave it enabled, then it scans in excess of 1000 items at boot (which slows things as avast scans the files it accesses during boot). It is also resident, using system resources.
Not to mention the problem your are experiencing with unencrypted signature files, which is a bumbling oversight. So my suggest would be to use one or more of the on-demand anti-spyware options, disable the PO anti-spyware plug-in and speed your boot times.