With all the problems of that other OS, it is hard to choose what is the worst:
- DRM
- Bloat
- lock-in
- lack of interoperability
- cost of licences and maintenance
- malware
In terms of costs malware has to be one of the biggest. Some folks pay more to delouse that other OS than to license it.
A recent report from Panda suggests that 59% of PCs are infected. Viruslist shows there are a thousand times more malwares for that other OS than for GNU/Linux. Even if it were only the ratio of malware that mattered, it would be smart to run GNU/Linux. We know it is more than that because most of the attacks on systems running GNU/Linux are rootkits resulting from weak passwords. That other OS has so much complexity that finding vulnerabilities in it is a large, growing industry.
If people must use that other OS they should send the bill for securing it to M$.

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