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I am making request to my asp page as

http://localhost/test/test.asp?id=mike

this works great query string value is retrived as "mike"

whereas when I give the URL like this

http://localhost/test/test.asp?id=%aaa

I dont get id value = %aaa

instead I get some garbage value...

is there some kind of encoding required when % is involved in querystring...

Yes, non-alphanumeric characters need to be encoded to travel in a URL. If
the link is generated by script, you can use the command:
server.URLEncode("%aaa")

P.S. it looks like you might be passing query string variables directly to
SQL queries, which is very hackable and insecure. Rule 1 of web programming
is to validate all variables from the client You might try:
?searchtype=endswith&searchfor=aaa
and after checking the searchfor string for unexpected junk, construct the
"%aaa" on the server side.