We are working with paypal payments standard in our website. When users fill up their request in our site and press the paypal button \"Buy Now\" we submit all the variables to
Based on the other answers, comments, and votes to close, I think there is some confusion.
There are two legitimate options for users. Either enter payment information or login with an existing account. This can be somewhat confusing for users, who may or may not know/remember their login information. Last I checked, you do not need a PayPal account to use PayPal Standard payments (although I do believe CC# and contact info are checked against existing accounts if you don't login).
To answer the original question, I don't have first-hand experience overriding this behavior, but it may be possible.
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_SetExpressCheckout
(See the LANDINGPAGE parameter)
(Optional) Type of PayPal page to display. It is one of the following values:
Billing – Non-PayPal account Login – PayPal account login
Paypal will always lean towards encouraging existing users to login (which makes sense from a marketing/retention perspective and also from a security perspective).