Write to Common Temp?

[poi]

I have ASP.NET users using Windows authentication, impersonation is on.
I try to write a temp file (large persisted dataset) and just keep the
file path in an in-memory session variable. It bombs because the temp
file ends up written to the ASPNET user's temp directory, where the
impersonated user has no permissions.

How can I write to something lilke %windir%\temp or something writeable
by everyone?

Thanks.