Have you seen this user-agent string?

In my web server logs, there appear to be a particular user-agent coming from hundreds of different IPs, with requests always being ("GET / HTTP/1.1"), and referrers being empty. A quick search for some of the IPs on google turns out that they are on various spam blocklists. I am thinking that maybe this is a botnet visiting people's websites? I wonder if anyone else noticed this.

Here's the "exact" user-agent string I'm referring to:

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)