Security vendor NoVirusThanks has released SSDT View, a 64-bit (only) tool that can show you the contents of your System Service Descriptor Table, perhaps highlighting changes made by rootkits and other stealthy malware.
There are of course plenty of antirootkit tools around that can do something similar, and a whole lot more, but these are generally aimed at Windows experts. SSDT View is safer, and far simpler, which makes the program accessible to a far wider audience.
What’s the SSDT? Whenever Windows or one of your applications wants to carry out some action -- check the Registry, read or write a file, launch or close a process, and so on -- then this will usually result in Windows calling a service in the System Service Descriptor Table. Writing to a file will call the NtWriteFile service to do the actual work, for instance; on our test PC that entry points to memory address 0xFFFF:F800:0356:B210, which is within the module C:\Windows\system32\ntoskrnl.exe – the Windows kernel.