It is hard to say that one AV, Trojan scanner, etc. is best, because there is not one that will detect all problems.

Some tips here: http://reviews.cnet.com/5208-6132-0.html?forumID=32&threadID=1313&messageID=15002

You might consider installing Mozilla / Firefox.
http://www.mozilla.org/

Spyware detection and removal programs:
You may also want to consider installing either or both of AdAware (free version) and Spybot S&D (freeware). Use these programs to regularly scan your system for and remove many forms of spyware/malware.
a. AdAware: http://www.lavasoft.de/software/adaware/
b. SpyBot S&D: http://security.kolla.de/index.php?lang=en&page=download
Check for updates in Adaware frequently.
I would check for updates in SpyBot once a week or so.
I scan with each at least weekly.

Trojan scaners, to name a few:
TDS-3 /DiamondCS
http://tds.diamondcs.com.au/ ** Download & install the 30 day free trial, update it manually as described here: http://tds.diamondcs.com.au/index.php?page=update as the trial version doesn't have auto update enabled so you will have to do the update manually.
The automatic update only works with the registered version which costs $49.

A2 squared free Trojan scanner:
http://www.emsisoft.com/en/software/free/

TrojanHunter:
http://www.trojanhunter.com/ Also has a free trial version.

The Cleaner:
The Cleaner is free for 30 days http://www.moosoft.com/products/cleaner/download/

There are also various online Trojan scanners:
http://scan.sygatetech.com/pretrojanscan.html
http://www.windowsecurity.com/trojanscan

Also, the following free programs:
a. SpywareBlaster: http://www.javacoolsoftware.com/spywareblaster.html
b. SpywareGuard: http://www.javacoolsoftware.com/spywareguard.html
Periodically check for updates.