Problems with IE window.open and validation controls. Works fine with Mozilla

[Lord of Chaos]

hi,
I m having problems with validation and IE windows.open.

here's my website
http://www.invoicernow.com/Invoices/Add.aspx

account: testms
pwd: 1@a

basically when windows.open is called, it also calls validation and doesn't
let the popup window open. however, the same site works in IE.

recreate the senario, goto my website
http://www.invoicernow.com/Invoices/Add.aspx

use the above account and password.

In the Customer Id field, click the search button, you will see, it works
perfectly fine in mozilla which opens a popup, but in IE a new popup window
doesn't open and only validation errors are reported.

so, what's the fix?
thanks in advance.

-suresh