An invisible keylogger is malware which attempts to record your keystrokes and send the details 'home'.
For example it may be able to log the keystrokes you use for your PIN and password at your online banking site/s and them send these details back to The author of the keylogger. S/he can then replicate these strokes to get into your online bank account.
Suggest that you turn off System Restore, update all of your AV and malware scanners and then run full scans. Don't forget to turn Sys Restore back on when you've finished.
By disabling Sys Restore your scanners should find any 'nasties' caught in memory between Restore Points.