It doesn't work if checksummed on a file-by-file basis: if the file is
large enough you can DoS-attack the system just by changing a single byte
every so often (oops, the whole file has to be reread...)
Checksummed blocks *do* work, and are useful to detect a variety of
disk-incurred problems like writes that accidentally landed in the wrong
place and so on. But this doesn't help to detect malware because malware
is using exactly the same syscalls as non-malware. If used for that
purpose it becomes like the UK ID card, where what they really want is an
`I am not a terrorist' card...
What Talpa is doing, instead, is using what amounts to a huge mugshot
database of known bad guys. Unfortunately for them there's no right time
to check the data written against the mugshot, and whatever method they
use the next generation of malware will specifically evade...