As well as what Shiraz said, try something like this:
http://<server>/ReportServer/Pages/ReportViewer.aspx?%2f<path>%2f<ReportName>&rs:Command=Render&UserID='fred'
Note the path would only work if you are in a single folder. When I have to do this I simply browse to the report using the reportserver path ("reports" is the report manager) and copy the url then add &<ParameterName>=<value>
to the end.