Should I install software sent to me via Spam?

In a nutshell, the answer is NO. But to fully answer why, you need to understand what Spyware is.

Spyware is basically programs that install and run on your computer (sometimes with your consent - part of a game, or screensaver, password keeper (Gator) or other applet doing the rounds where you agree to their terms- or sometimes without your consent, just by surfing around the internet). They have a variety of functions. Some are relatively harmless/ mildly annoying. A fair number keep track of where you go and pass this information on to unknown parties. A fair number also hijack your browser, change your home/search page, take you places you don't want to go, give you millions of popups/messages. And a tiny number are downright evil and steal your passwords and go looking for your credit card numbers, which they pass on, again to parties unknown. Sometimes they are the precursor to turning your machine into a "Zombie" machine.

With anti spyware programs that come to you via Spam, they often make claims like "Guaranteed to get rid of XXXX, YYYY, ZZZZ spyware programs".
And they do.
What they don't tell you is that XXXX,YYYY and ZZZZ are their competitors, so they will get rid of their spyware, BUT they will install their own versions which do similar jobs (or may be far worse and may be much more difficult to remove) on your PC now that it has got rid of its competitors.

Spyware has also been known to come in via Sponsored Links (see the link on the home page), so be careful there too.....