You need to clear your cookies/cache
In the first instance please try this link and click Yes
ucp.php?mode=delete_cookies
If that fails, you will have to do it manually.
To do this in Internet Explorer goto Tools > General tab > Browsing History > click Delete... > Cookies ticked > click Delete or OK button. Restart IE and try to log in again.
ADDITIONAL NOTE
Tested this on firefox to replicate the problem and this process fixed it for me.
when in firefox, click Tools and then "options" from the list.
Select the privacy tab (little picture of a mask)
Select remove individual cookies from the centre of that box.
find 350z-uk.com in the list and select it (click once so it selects that folder of cookies rather than expand the whole list).
Click remove cookies.
log back in again and all should be well.
Another thing that may sort your login problems is this piece of software which is ccleaner you can download it from here. No pc should be without it imho.
http://www.filehippo.com/download_ccleaner
Cleans the following:
•Internet Explorer
•Firefox
•Google Chrome
•Opera
•Safari
•Windows - Recycle Bin, Recent Documents, Temporary files and Log files.
•Registry cleaner
•Third-party applications
•100% Spyware FREE
Feedback will be appreciated if this helps in anyway.