Unfortunately I could not replicate this on my end, even with Windows XP running Internet Explorer 8 and using a test account.
There are two possibilities I can think of:
Are you using a proxy or satellite Internet service?
To check if you've a proxy set, go into your Internet Options (Tools menu), then go into the "Connections Tab" and "LAN Settings". The "Use a proxy server..." option should not be checked.
If you've recently switched to Satellite Internet, there's a possibility it may be causing the issue. To overcome the long latency issue with a satellite connection, some providers have a server that fetches all the images for a page remotely and sends the lot in one go over satellite to cache on the user's satellite box so that as the user's browser fetches the individual images, they are loaded instantly. The problem here is that when the remote server fetches the images, Myce's server probably sees that server as a guest and thus returns the message about not being logged in and also about having 0 posts.
Do you have any plug-ins that may be causing a problem?
A simple way to check is to try browsing Myce with add-ons disabled. To do this, go into Start -> All Programs -> Accessories -> System Tools -> Internet Explorer (No Add-ons)
If attachments display fine in this mode, then there is a plug-in or toolbar that is causing a problem.