How do I am sure that no keyloggers are installed on my system

Keyloggers register every letter you type on your keybord or interaction you do and even takes screenshots. Some keyloggers only work when you are visiting certain banks, financial institutions, others take only passwords for all or some specific interactions. These passwords are guarded in a textfile which is sent to the malwarewriter.
You can use Keylogger Hunter is a program that attempts to detect any keyloggers that may be running this is the last freeware version http://freeware.it-mate.co.uk/gt.asp?t=http://freeware.it-mate.co.uk/downloads/klhuntersetup.exe

But you should not be fooled. This program will not protect you against buildin hardware keyloggers or some technology that has find a way past this.
xp only Snoopfree is a good free anti-keylogger that may detect the password stealer and any other keyloggers that could be hidden on your computer. http://www.snoopfree.com/ Another nice program for finding keyloggers is Security Task Manager http://www.neuber.com/taskmanager/index.html while it is not a free program, it is very good at finding keyloggers. It does have a 30 day free trial too commercialware http://www.privacykeyboard.com/
One protection you can already take is never to save passwords on your pc.

Another nice program for finding keyloggers is Security Task Manager http://www.neuber.com/taskmanager/index.html while it is not a free program, it is very good at finding keyloggers. It does have a 30 day free trial too
commercialware http://www.privacykeyboard.com/

Hardware keyloggers used to be easy to identify and remove, see here for a description http://www.spycop.com/keyloggerremoval.htm But now hardware keyloggers can be hidden within the keyboard itself, or within the computer case, so it is very difficult to find them.