Hi,

     I was installing WSUS v3.1.6001.65 and the installation kept failing just as it was copying files, the error I was getting was quite a strange one in the installation logs:

An exception occurred during the Install phase.
System.FormatException: Input string was not in a correct format.
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)

 After many hours of research it turned out to be an issue with the performance counter names/drivers that caused the stange issue, I uncovered white paper talking about WSUS and mentioned that re-registering the counters may help.  It linked me to the MS web page: http://technet2.microsoft.com/windowsserver/en/library/8ca82333-eca8-413b-b8c8-c6defad6de3e1033.mspx?mfr=true

Which gives a command of LODCTR (found in the system32 folder, using the following syntax seems to have fixed my issue:

 C:\WINDOWS\system32> lodctr /R

 The command does take a little time but once completed, I just tried to install WSUS again and it went straight through!.  

 I figured that it may help someone