How can i define urlRewrite rule

 Hi
I used  RewriteURL  method to get this
www.mysite.com/products/1/mobile/demo.aspx
instead of www.mysite.com/products/demo.aspx?productid=1 & product = mobile
written the following Rewrite  rule in Webconfig file

 <rewriter>
    <rewrite url="~/(.+)/(.+)/Demo.aspx" to="~/Products/Demo.aspx?ProductId=$1&amp;Product=$2"/>
</rewriter>
design page have this link <a href="<%#Eval("ProductId")%>/<%#Eval("ProductName")%>/Demo.aspx">BUY NOW</a>

Everything is working fine
when i pass the any of querystring with space like product = mobile phone instead of product = mobile (simply added space)
then my url gets www.mysite.com/products/1/mobile%20phone/demo.aspx
clearly the space replaced with %20 but i dont want to show %20 in my url
how can i do that
wheth ...