Windows Server 2008 R2 doesn’t include the Disk Cleanup tool by default.   Either install the Desktop Experience feature, which requires a reboot, or copy

C:\Windows\winsxs\amd64_microsoft-windows-cleanmgr_31bf3856ad364e35_6.1.7600.16385_none_c9392808773cd7da\cleanmgr.exe

to

%systemroot%\System32

and

C:\Windows\winsxs\amd64_microsoft-windows-cleanmgr.resources_31bf3856ad364e35_6.1.7600.16385_en-us_b9cb6194b257cc63\cleanmgr.exe.mui

to

%systemroot%\System32\en-US

To start Disk Cleanup type cleanmgr.exe at the command prompt.