It depends on how good the software is to start with. One discouragement I had over 13yrs. ago was when choosing the wrong one! (Not to name McAfee! ha ha)
Over the years I tried out the various shareware downloads as well as free for home versions and always ended up running at least 3 or 4 which at times included things like IE security addons. The AVG toolbar option and later the Crawler Security toolbar for IE are a few examples.
When comparing another program when adding along with AVG I was finding the Spyware Terminator was finding far more data miners and other bugs faster then AVG which seemed to miss the minor ad/spy bots and other things while AVG had sprung to life to flag a bug coming from one bad site and Avast failed! I kept running into the need for those other extra programs to fill in the gaps regardless of which main program I had on!
For the purchase of VIPRE in May 2010 for a two year license some would think the price might be a bit high until finding out what I was seeing here with VIPRE finding what every other program tried out and ran over several years has missed! Cleverly hidden malwares in what was supposed to have been free utility programs for XP!
VIPRE was found to have the highest detection capability. And the reviews generally would be looking mainly at programs that have been on the market longer seeing regular newer versions but not built on an updated platform for an av program.
When looking at a retail purchase rather then going for bust simply grabbing the free version that comes along for an av program this would be something to look for whether it was VIPRE or any other paid for software. In your circumstance there flashdrives you are in an office environment where email scanning/detection of worm types viruses can be accomplished by any of several av and email security programs along with a good firewall.
As for the home user unfortunately the security measures are not always inplace and people get "too comfortable" not even realizing the malwares are out there to be stung by! If you are using rewritable media or other options for transferring data between machines without a means to detect and block any malware being spread then you have to look at what is more costly in the long run. The 1-2yr. typical software license? or the downtime you would find yourself facing in particular if the office machines were infected?!
For the novice home user without experience at removing bugs or even installing an OS where does that leave them? The local pc repair shop might just charge "an arm and a leg"!

Meanwhile there was one other thing I should mention here I had forgotten to mention before about zeamann's trouble when trying to install the main program. You could try running the stand alone VIPRE Rescue Program if you are still stuck with the malware situation there.
VIPRE Rescue - VIPRE Computer Recovery Solution from Sunbelt Software
You can download that directly to a flash drive and when running it the tool will create and run out of a temp folder without any new registry values to get Windows back running again. On one machine I had an owner boot up in safe mode long enough to create a desktop shortcut when a virus created a new admin account locking him out.
The VRP took care of the I-Worm infection and gave him a normal desktop again but unfortunately the worm had already done more then first thought. But when seeing Windows running normally again on any other better circumstance then you would run the 30 day in order to see the drive(s) sweeped totally of any traces.