Have you tried cl_allowdownload commands?
cl_allowupload
or
cl_downloadfilter
?? These being used if its permitted that is.
also you might want to check your internet restrictions. Firewall up? Router acting screwy?
Do a full analysis of your problem and write it here, then im sure zero or another who knows more then i do can help indefinitely.
edit: BUT before you even concern yourself with ANY of this... Try just Deleting your entire cstrike folder, restart steam, then open CSS again.