Startcom is great (I use it for a bunch of domains), but there are some caveats:
* They are only free for personal, non-commercial use. You can't (contractually) use them for your startup.
* They are free to acquire, but they charge to revoke them. So don't lose your key (even accidentally, via something like heartbleed).
* Their roots are not in the Windows XP base install. They are included in an optional update, but my experience with a web site that had old machines in its demographics showed that practically no one had it installed. Given that XP is no longer supported by Microsoft, this point is getting less and less relevant as the days go by.
* They are only free for personal, non-commercial use. You can't (contractually) use them for your startup.
* They are free to acquire, but they charge to revoke them. So don't lose your key (even accidentally, via something like heartbleed).
* Their roots are not in the Windows XP base install. They are included in an optional update, but my experience with a web site that had old machines in its demographics showed that practically no one had it installed. Given that XP is no longer supported by Microsoft, this point is getting less and less relevant as the days go by.