IE:
Tools -> Internet Options
Click on the "Privacy" tab
Click on "Sites..."
Enter your domain name in the box and click "Allow", then click on "OK"

Firefox:
Tools -> Options -> Privacy
Click "Exceptions" under the Cookies section
Enter your domain name in the box (both with and without the www. on it) and click "Allow"

If that doesn't work, tell her to try deleting all cookies from your site and try again:

IE:
Start -> Run
When the dialog box pops up, type %USERPROFILE%\Cookies
Delete any files with a name of {her_username}@{yourdomain}[somenumber].txt or {her_username}@www.{yourdomain}[somenumber].txt

Firefox:
Tools -> Options -> Privacy
Expand the Cookies section
Click on "View Cookies"
Hold down the CTRL key and click on anything where the "site" is either "yourdomain.com" or "www.yourdomain.com"
Click on "Remove Cookies"

Notes:
%USERPROFILE% = shortcut to your documents and settings folder
{her_username} = whatever username she uses to login to Windows with
{yourdomain} = your domain name minus the .whatever (.com, .net, .org, etc.)
somnumber = an auto-incrementing number that Windows uses to differentiate between multiple cookies from the same site