cmd/release: make Windows installer require WinXP SP2+

Instead of Win2k.

Fixes golang/go#16544

Change-Id: I6068e402428a7cc65f18e4d236801ce609a73d56
Reviewed-on: https://go-review.googlesource.com/33354
Reviewed-by: Quentin Smith <quentin@golang.org>
diff --git a/cmd/release/releaselet.go b/cmd/release/releaselet.go
index b1021ae..b088fec 100644
--- a/cmd/release/releaselet.go
+++ b/cmd/release/releaselet.go
@@ -562,7 +562,9 @@
 <Icon Id="gopher.ico" SourceFile="images\gopher.ico"/>
 <Property Id="ARPPRODUCTICON" Value="gopher.ico" />
 <Media Id='1' Cabinet="go.cab" EmbedCab="yes" CompressionLevel="high" />
-<Condition Message="Windows XP or greater required."> VersionNT >= 500</Condition>
+<Condition Message="Windows XP (with Service Pack 2) or greater required.">
+     (VersionNT >= 501 AND (WindowsBuild > 2600 OR ServicePackLevel >= 2))
+</Condition>
 <MajorUpgrade AllowDowngrades="yes" />
 <SetDirectory Id="INSTALLDIRROOT" Value="[%SYSTEMDRIVE]"/>