JavaScript for checking user

[noor]

hi,
can any one tell me a javascript that can be called on mouseover event of a
html link control .
script can check from session either a user is login or not
In the case of Login it will redirect to the given url.
In the case of not Login it will prompt the user that u r not login kindly
login.

actually that link leads a user to Download Software. but this feature is
only for registerd user.

kindly help me. it will be best if u give me a script for it.
Thanks
Noor