The problem you're having is that Username is a string field, so you need to quote your field:
"SELECT Username, Password FROM tblLogin WHERE Username = '" + txtUIserName.Text + "'"
BUT NEVER, EVER build SQL expressions using concatenation. Sorry, but that isn't very smart at all. What if I pass "blah' AND 1=1; drop table tblLogin" from your TextBox? Bye bye login table. It's even worse that you store passwords in plain text. I could, instead, select all those and impersonate any user on your system. Can you say "lawsuits"? Read http://www.codeproject.com/script/comments/forums.asp?msg=932507&forumid=1649&XtraIDs=1649&searchkw=parameterized&sd=7%2F8%2F2004&ed=10%2F6%2F2004#xx932507xx[^] for other problems and more things I could do using your SQL concatenation code. ALWAYS use parameterized queries. Read the link above for an example, or the OleDbParameter class documentation in the .NET Framework SDK. Also, your expression is very insecure besides not using parameterized queries. Don't select the password as part of the result set. Send the password and use it in the WHERE clause like so:
SELECT COUNT(*) FROM tblLogin WHERE Username = ? AND Password = ?
In fact, even that's bad. You should never store passwords in plain text. Hash them using a one-way algorithm like MD5 or SHA1, both of which are supported by the .NET Framework base class libraries (see the MD5 and SHA1 class). Hash the password before sending it to the database server (and I assume this is actually for an ASP.NET application, which belongs in the ASP.NET forum but I'll continue anyway) or across the wire, then compare that to the hash you store in the Password field (using the same query as above). In fact, even that's insecure. It's subject to replay attacks where someone sniffs your password and while they might not be able to unhash or decrypt it (at least with a given amount of time and resources), they could simply capture the packets, play them back, and you've been 0wned. Instead, store a salt, send it to the client, hash your password + the salt value and s