I have deleted all invalid entries with Regseeker & RegSupreme for years I don't think it has ever causaed me any problems. I do believe it's one of the things that helps keep my pc's almost trouble fee.
I have found that my free copy of Ashampoo keeps resgistry and Hard disc nice and free from rubbish. The free copy was on PCADvisor CD a couple of years ago but the web site will let you download a trial version. See if that helps.
I've used RegSeeker for a while now and always delete everything without ever having a prob. Do as MichelleC says and backup your registry first, RegSeeker will also make it's own backup of the changes it makes. 408 invalid entries is reasonable for the first run of a registry cleaner.
"Understanding the Windows Registry Your system's registry is a central database used to store information necessary to configure the system for preferences, applications, and hardware components. Registry repair and preventative maintenance is important because the registry contains information that is constantly referred to by the system during operation. The more preferences, software applications, drivers, and hardware components that are installed on the computer, the more complex the registry becomes. Registry repair becomes critical when this registry becomes corrupted or littered with keys no longer relevant to your system. The Windows registry is a vital part of Windows and if it is damaged then your whole system may be corrupted."