It's an automated spam bot trying to create an account so that it can post spam on your site.

Best way to deal with it is to install the Mollom module.

http://drupal.org/project/mollom

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Completely my own fault. I quickly set up a non-profit site and stupidly I left an option in forums for anonymous users to upload content, without administrator approval. As the site contained mostly static information I did not look at it for maybe two months ( I hear you groan ) and lo and behold I had more than a thousand nodes of the usual junk ranging from porn to pills. I started to delete them, but what a job, so my solution was I completely wiped the site and the database, and made a new installation, even with a new ip address.

One of the first things I did on the new sites is I made sure that EVERYBODY had to logon to post comments or content, secondly, I installed Mollom ( thanks Dries!) , and then the Security Review module. However, this did not stop the automated spambots from attempting to login to the site, as evidenced in the site log. It was simply incredible, and also the 'page not found error' up to 3-4 times a minute expecting to find the junk that was already there. The 'hit' count was incredible! I then decided to ban their ip addresses using 'access rules' & I applied it to any address that was obviously a spambot, plus those guys who tried to be clever and type in such things as /php, /css, /user/user, the list is endless...Eventually I've banned about 35 ip addresses, often using % as many of them have multiple addresses. Out of curiosity, I looked up a lot of the addresses and found most came from russia, followed by the old 'east bloc' , a particularly active one in Holland who I actually sent an email to, China & the states. Now I check my log files everyday, to protect the site and secondly I feel vindictive!A lesson learned, luckily on a non-profit site & the damage was quickly fixed, and in fact just installing 'captcha' onto everything that can be posted will cut out quite a lot of stuff.

check out http://buytaert.net/ten-ugly-comment-spam-techniques boz

I then decided to ban their ip addresses using 'access rules' & I applied it to any address that was obviously a spambot, plus those guys who tried to be clever and type in such things as /php, /css, /user/user, the list is endless...Eventually I've banned about 35 ip addresses, often using % as many of them have multiple addresses. Out of curiosity, I looked up a lot of the addresses and found most came from russia, followed by the old 'east bloc' , a particularly active one in Holland who I actually sent an email to, China & the states