[NinetyNine]

I can't verify this, but unless you installed Helvetica manually, windows installations won't have it and will fall back to whatever is specified next in the family (usually Arial or the default Sans Serif).

[achal]

Which is even worse; Arial looks terrible compared to Segoe UI or Calibri.

[vetinari]

Some printer drivers install Helvetica, usually badly hinted (I'm looking at you, HP).