Sorry, I'm no admin or mod, but I may be able to help.
It sounds like a cookie problem. When you are posting, does it show you as logged in? You can see at the top left it says Welcome, .........
You can try deleting cookies and your internet cache, but you will lose all other saved passwords, etc.
What browser (and version) are you using? Have you tried logging in with a different browser? If you use IE7, try Firefox 3 and vice-versa.
Having just done a quick internet scan, I notice a lot of people who have forum login problems use IE7 and claim it is still pretty buggy. If thats the case remains to be seen, some other members here no doubt use IE7 too so hopefully somebody could add some light to this? I remained with IE6 but also use Firefox 3 and have no problems whatsoever.
See if there are any patches for your browser, or upgrades... or simply use another browser.
Hope that helps
