Write down what it is looking for, the DLL then go to Run and type Regedit press enter key, go to Edit in Menu click Find the type in box the DLL you are looking for. Make sure it is exact. press enter the delete the one it finds press F3 to go to next until it as all gone. Do not remove anything else
Try using Autoruns from Microsoft install it and run look in the listing for that exact, name if you find it unticking the box to the left of the entry then closing Autoruns and rebooting should fix your annoyance
Yes, it could be hidden in a restore point. To make sure it's totally removed, turn system restore off, run Avast to see if it's picked anything up. Once the scan is complete and you know your systems clean, remember to turns system restore back on!!