Avast is not a spyware scanner, it is an Antivirus program. There is a free version of Avast available (you get a license key every year) but if you had a 60-day trial, you must have chosen the paid version of Avast. That is quite good, too. (BTW, one of the reasons I like Avira slightly better than Avast is, Avira has fairly good malware protection, too.)
Stopzilla is, supposedly, an "Anti-Spyware program. Even the description suggests it is outdated, IMO. Comparing what it finds to Avast isn't an apples to apples comparison. There are very effective anti-malware programs that are far more effective (and far more honest) than Stopzilla. Two I recommend are Malwarebytes Anti Malware and SuperAntiSpyware.
Stopzilla has been suspect among security experts for quite a while. It has been accused of false positives and erroneous findings (basically making up detections to seem valuable. It is marketed using fake awards and is known to be difficult to remove.
Here's some discussion of Stopzilla, with the comment I find most accurate, below:
http://www.techspot.com/vb/all/windows/t-5...re-product.html
http://www.malwarebytes.org/forums/index.php?showtopic=1416
Hey, if you like it, it's your computer. But you should know it is considered to be a very ineffective product. Hell, it even gets a crappy rating (and comments) at download.com (I thought everything was rated highly there ):
http://download.cnet.com/Stopzilla/3640-80...html?tag=tab_ur