hahaa, there're already
many FUD [Undetectable!] Steam Tools, others .exes, etc. that are actially keyloggers/phishers, so even if you have 20 AntiVirus Softwares installed, all licensed and Up-to-Date, you
W O N ' T catch the integreated phisher into (because as I said, it's
FUD!). So ... don't type msgs like that - easy for attackers.
The seller
must give you the source code of the program and then you manually to compile (understanding what you're doing the verifing that there isn't any malicious writen code into, to be 100% secure). So you should be familiar with VB, C++, C#, VB2010, etc. software, not a n00b.
GL find that seller. 90% of sellers
won't give you the source code of the program, so... in the end - no "Steam Accounts Checker"

.