Cluster TEMP path on server MBXSERVER is set to the default location. For optimum performance, this path should point to a fast disk. Current TEMP path: %USERPROFILE%\AppData\Local\Temp – issue on Windows 2008 R2 and Exchange 2010 SP2

Exchange Best Practices Analyzer informs you about this issue and when you click the link offered it gets you to this web page: http://technet.microsoft.com/en-us/library/aa998044(v=exchg.80).aspx
There, the text offers you a solution to log in to a system as cluster user account.

1. Log on to a node in the cluster as the Cluster service account.
2. Right-click My Computer and select Properties.
3. Select Advanced.
4. Click Environment Variables.
5. In the User variables for <account name> area, select TEMP and click Edit.
6. In the Variable value field, enter the new path for the TEMP environment variable and click OK.
7. In the User variables for <account name> area, select TMP and click Edit.
8. In the Variable value field, enter the new path for the TMP environment variable and click OK.
9. Click OK to close the Environment Variables dialog box and OK once more to close the System Properties dialog box.
10. Restart the node to apply the change.
11. Repeat steps 1 through 10 for each node in the cluster.

!!! BUT !!! In Windows 2008 R2 (I did not check other versions of Windows 2008) cluster service runs as Local System and therefore this solution is not actually a solution. On web page http://support.microsoft.com/kb/243330 you can find known security identifiers which shows that Local system’s SID is:
SID: S-1-5-18Name: Local System
Description: A service account that is used by the operating system.
Now, open the registry and go to HKEY_USERS, expand S-1-5-18/Environment and change the TEMP and TMP values with the values you want.