I hate trial software, shareware or whatever you call it.
The idea sounds good in theory, you try before you buy. Sadly in reality it's a different story, if you install it, then decide you don't want it so uninstall it, there's always some crap left behind which often slows your machine down.
You can remove sh1tware completely without reformatting but it's a pain in the bum as you need to complete the removal manually and you need to know what you're doing. Once you've run uninstaller, search or files on your computer containing the program's name and delete them keeping a record of what you've deleted, then do the same with Regedit and you should've totally removed it. Failing that download, Autoruns, Process Explorer from Wininternals and use them to search for any hidden processes it may've left on the system, also Rootkit Revealer is a good idea too but now we're into the realms of looking for adware, in which case it's better to download some propper software fo that.
My advice is don't install it in the first place! I generally choose free software (as in open source) over proprietary software simply because of the honesty factor; if they're releasing the source code there's no way they'll put anything dodgy in there. Don't get my wrong, I don't have a probemlem with proprietary software. I'm just more careful when it comes to installing it. I always ask myself "Do I trust the vendor?" if not then there's no way I'll install it. If I semi-trust them I'll install it to a separate directory under a limited user account, that way it can't mess with any of the system settings.
I use a Knoppix live CD to format my hard disks with and I'd strongly recommend it, it's free and you can even resize partitions and create new ones. Knoppix also contains many tools to help you get damaged Winwoes installs working. You can change and modify any fileon your hard disk (which you can't do with Winwoes running) backup files onto usb sitck or CD (even if you can't get into Winwoes), and reset the password on the Winwoes administrator.