* fewer crappy apps running on startup
* improved startup time
* no unknown apps running after a clean boot
(Previous 3 can be monitored with e.g. SysInternals AutoRuns)
* no browser / networking stack hijacking extensions showing popups / interstitials / DOM modifications etc. at random intervals
(As evidenced by e.g. HijackThis)
* no unsigned DLLs / drivers of unknown provenance loaded in any process or the kernel
(As evidenced by e.g. Process Explorer's Verify Image Signatures option - look in loaded modules of the "System" process to see all loaded kernel drivers and modules), with a representative sample of applications running (browsers, productivity, etc.)
* no apparent rootkit-style hijacking of the process / registry / disk inspection routines, as would be shown by e.g. RootkitRevealer
And of course, a commercial virus scan, for what it's worth. I personally don't run a real-time virus scanner; I rely on monitoring what the system is doing directly.
Once a computer has become infected, it's fairly impossible to determine if the infection has been completely eradicated.[1] I wonder that that $19 buys you. A few executions of anti-malware software until they claim "Clean"?
[1] I realize that even in the absence of a known infection it's "impossible" to know whether you're infected.