Decrypting using another company's Public Certificate

Hi,

I'm on a project in which my asp.net page will be receiving a request
that has an encrypted query string attached to it. The encrypted
portion is actually the url but encrypted with the sender's private
key. So basically it looks like this:

(URL)(Encrypted URL)

My job is to take the sender's public key, unencrypt it, compare it
against the original url and make sure the url has not been tampered
somewhere along the way.

I've never had to do this before. Can anyone guide me to some
resources that I could study to understand what I'll have to do here?
I'm thinking I'll have to obtain the public certificate first then use
that to unencrypt it though I'm not too familiar with the
System.Security.Cryptography namespace.

Thanks!
Mark Moeykens