877-941-2552

I get a security alert message and it locks up my desktop until I kill IE. It tells me to call 877-941-2552. I had previously gotten this same type of warning, but with telephone  855-636-6675

Typical web browser hijack that was embedded in an ad on a website you visited. Almost certainly without the knowledge of the website owners since they do not control which ads appear via ad servers.  You should probably obtain a good malware remover and run it. There are several free ones available.  You can also see if your security software vendor has a removal tool on their website.  Windows Defender does not always catch these.  Once cleaned, you may want to try an Ad Block add-on for your browser. They seem to help prevent these hijacks from occurring by blocking ads from ad servers.