Ring3 API Hook Scanner is, just as its name suggests, a user mode tool which can reveal some types of hooks (inline, IAT, EAT) in processes running on your PC.
If that sounds horribly technical, then you're right: it is. But the underlying principle is very straightforward. Stealthy malware such as rootkits may intercept (or "hook") important Windows system calls to help hide themselves; Ring3 API Hook Scanner will look for this kind of interception and let you know anything it finds.
The program is simple to use. Just download, unzip and run either the 32 or 64-bit version, according to your needs (no driver is required). Click "Scan" and after a few seconds you'll see a list of any hooks found, with associated information, or nothing at all if your system is hook-free.
You can't conclude too much from this, of course. Having no detected hooks doesn't mean you're malware free. And some applications may use hooks entirely legitimately, so don't panic if Ring3 API Hook Scanner finds something.
If you're an experienced Windows user embarking on a manual malware hunt, though, the program could help point you in the right direction, so it's worth adding to your portable security toolkit.