After some research I came across Microsoft KB2661254 which mentions this is due to the minimum key length of the certificate being less then 1024 which in this case it will be blocked. This Microsoft KB article can be found on the following site:
After reviewing the default VMware certificate generated on the vSphere server, I verified that the certificate key length was only 512 as shown in the following screenshot.
As a result following the advice from Microsoft on KB2661254 I lowered the required certificate key length to 512 using the following command from an administrative command prompt:
certutil -setreg chain\minRSAPubKeyBitLength 512
After making this change, I was able to continue past the certificate warning as normal and download my vSphere client.