You say that, but it depends on what you install.
If you stick to good software and avoid warez, free trials, shareware, screensavers and desktop themes then generally you'll be alright.
Some free trial and shareware programs don't uninstall properly, they tend to leave a little something behind that tells the installer not to install it again. Screensavers and desktop themes are a well known source of malware so aviod them at all costs unless you're absolutely sure.
When you're searching for freeware to do a particular task, then try to stick to open source software where possible and download it from sourceforge or the developer's site and compile from source if you can. If a program is open source, then there's little chance of it being malware because all the code it open so any rubbish can be removed.