OK Here goes ...
Let's say someone say 'You should use TWADDLE its a great utility to ...'
If I know and trust the person I may skip some of these steps but if I don't know them and have never heard of twaddle ...
Read user reviews - if there aren't any I am already suspicious
Go to the download site for TWADDLE and see if any reviews are given by users (not the developer's blurb)
See if it is downloadable anywhere else (Google "TWADDLE download") and see if reviews are posted elsewhere
Do a google search on the app name + Review
Look for specific reviews Google "TWADDLE review", you could also try looking for the type of software to see if there are other more favoured alternatives.
I find it is often useful to do this initial trawl as it not only gives a general impression of user response to the software but helps you to identify the current version. There are often multiplte versions out there and not all download sites are efficient at ensuring you get the latest version.
Do a google search on app name + Spyware/AdWare/Malware etc.
See if there are any known issue with the software Google "TWADLLE spyware" for example will turn up references on sites relating to anti-spyware products. If you don't turn up specific problems then you are unlikely to encounter spyware problems (but it isn't guarnateed).
Download the software and virus scan
Download to your hard disc and run your virus scanner to check it. I use NOD32 anti virus, so I just right click on the download and hit the appropriate link to virus check it. You can also use download managers (such as GetRight) that can be linked to your antivirus program so that all downloaded files are scanned automatically.
Unpack the archive/installer and virus scan again
Most downloads come insome sort of compressed format (such as Zip files). Many of these can simply be unpacked to a folder on your hard disc. If so do this and virus scan the folder agin (sometimes files in archives can be missed by virus scanners before they are unpacked)
Use SpySweeper to check the unpacked folder contents
I use SpySweeper to detect spyware etc. so I run a quick scan on just that folder to see if there are any recongnised files (such as DLL files) which may cause a problem. If something turns up I investigate what the problem file is (usually using Google). Often they are simply common library files which are of no threat in themselves but over zealous anti spyware programmes flag them up as problems (I found CounterSpy was esp. bad at this sort of false positive).
Set a system restore point
You can do this in Windows XP (and Windows ME if you are still daft enough to use this). This simply takes a snapshot of important system files and registry settings so that if something goes wrong you can restore the system to its previous setting. To do this (on WinXP) go to Start>Accessories?System Tools>System Restore.
Install app but don't run it
Run virus & spyware check on installation folder, memory and registry
If you are still happy run the installer application but don't run the programme when installation has finished. Some files may have been packed in password protected archives within the installer package and therefore couldn't be checked by you AV software. Run you antivirus software and antispyware on at least the installation folder (usually c:>program files>TWADDLE or similar) and also on C:\Windows.
Only if it is still clean do I run the app.
If it unexpectedly asks to connect to the internet I check to see if there as automatic update and switch it off. If it still wants to connect to the internet for no good reason than I block it . Anything odd starts to happen it is uninstalled and the system restore point applied. Am I paranoid? Probably - but it takes a hell of a lot of hours to sort out a screwed up system ....
Don't forget some software will NEED to connect to the internet - but whether TWADDLE needs to is a matter of judgement, and to a certain extent trust at this point.
Now go and have a strong drink and take a couple of the little pink pills ...