We all know the difference between a limited user and an administrator user under Win2k/XP - you can't/can install major software, perform system maintainence, and other stuff. But using a limited user on a day-to-day basis also provides you with decent protection from a bunch of threats: if the malware is running under your limited-rights user, it can only do as much as you can. For instance, a limited rights user can't edit the HKLM hive of the Registry, so any malware running under the same user won't be able to touch that area. It's extremely simple to implemnt, and pretty effective.
Note: don't forgo the anti-virus and Windows updates just because your user is slightly protected.

i think it is pretty uncomfortable, to be changin users, expecialy whith my slow pc

Here's a trick to work with my security method: If you need to run an executable (say, an installer), right click on the file in Explorer (or on any shortcut) and choose Run As.... You will get a dialog box asking you to enter an admin password. Supposedly Windows Vista will do this for you, but let's not discuss that.....

I always use the RunAs command. It DOESN'T work on the Internet/E-mail programs on the start menu (although you can do it on those programs, just not there). Microsoft Office programs can't do that (you can do it in Program Files).

This is a good security tip.

Can't a malware that captures keystrokes or saves critical data use the RunAs command to abuse it ?

Ok, if you're on a school computer, here's a tip to run whatever you want... just name it the same as an existing program (e.g. notepad.exe, iexplore.exe) and it will run... It is really nice and you can do stuff like run Firefox on school computers. (Portable firefox is really nice at school.)

for the slower pc - log-off first before you change to another account . it is safe to say Run As is handy but can be as sluggish too at times

Hm..very good advice. Though I am a bit sceptical because if you know that, hackers know that also almost for sure so they make such programs that use the runas command or something similar that bypasses this limited account...However, this is still a good way to protect youself if you don't mind switching between acounts when trying to install a new program or something...

Greetz

They can't make their programs use runas because most people should have a password on their administrator accounts (whether it's one they made, or the one windows has by default). To change the passwword of the administrator, log on as an administrator, go to start -> Run, enter control userpasswords2 and hit enter, click on Administrator in the list, and click Reset Password.