Hi, welcome to the forum 
I can’t say for certain but as it’s calling [Convert]::FromBase64String and as we have no other information from you, my guess would be you’ve picked up some malware. It’s often Base64 encoded to obfuscate what it’s trying to do.
It’s beyond the scope of this forum to provide advice on scanning for and cleaning-up malware.
You may try the /r/techsupport malware guide:
/r/techsupport itself (they’ll probably refer you to the guide above).
Or the BleepingComputer forums: