How to specify the platform for MSBuild

I am trying to use MSBuild to build a solution with a specified target platform (I need both binaries, x86 and x64). This is how I tried it:

C:\WINDOWS\Microsoft.NET\Framework\v3.5>MsBuild SolutionPath\Solution.sln /t:Rebuild /p:Configuration=Release /p:Platform="x86"

However the build always fails if the platform is different from "Any CPU". What am I doing wrong?

This is the while output MSBuild prints:

C:\WINDOWS\Microsoft.NET\Framework\v3.5>MsBuild
SolutionPath\Solution.sln /t:Rebuild
/p:Configuration=Release
/p:Platform="x86" Microsoft (R) Build
Engine Version 3.5.30729.1 [Microsoft
.NET Framework, Version
2.0.50727.3082] Copyright (C) Microsoft Corporation 2007. All rights
reserved.

Build started 1.7.2010 8:28:10.
Project "SolutionPath\Solution.sln" on
node 0 (Rebuild targe t(s)).
SolutionPath\Solution.sln : error
MSB4126: The specified sol ution
configuration "Release|x86" is
invalid. Please specify a valid
solution c onfiguration using the
Configuration and Platform properties
(e.g. MSBuild.exe Solution.sln
/p:Configuration=Debug
/p:Platform="Any CPU") or leave those
prope rties blank to use the default
solution configuration. Done Building
Project "SolutionPath\Solution.sln"
(Rebuild t arget(s)) — FAILED.

Build FAILED.

"SolutionPath\Solution.sln" (Rebuild
target) (1) ->
(ValidateSolutionConfiguration target)
-> SolutionPath\Solution.sln : error MSB4126: The specified s olution
configuration "Release|x86" is
invalid. Please specify a valid
solution configuration using the
Configuration and Platform properties
(e.g. MSBuild.ex e Solution.sln
/p:Configuration=Debug
/p:Platform="Any CPU") or leave those
pro perties blank to use the default
solution configuration.

0 Warning(s)
1 Error(s)

Time Elapsed 00:00:00.03

If I try to build it for x86/x64 with devenv it works perfectly, however I am trying to set up a build server without installing all the necessary versions of Visual Studio. By the way, if there is a better free tool (that supports .NET framework 4) out there, I'd love to hear about it.