The "administrator" account is a prime target for ne'er-do-well's who may wish to try their hand at compromising your system. The account name is a "given", leaving only the password to find, and if you haven't set a password on the admin account, well how easy do you want to make things? The admin account, unlike any others, does not get "locked out", no matter how many incorrect logon attempts are made.
Paranoid? Maybe, but would you like to take the chance? The default admin account created during windows install has many special permissions. I'm not exactly sure what these are, but it's odds on that your local neighbourhood script-kiddie does. Read almost any windows security advice site and they'll tell you, not only to not use the default admin account for everyday usage, but to rename it, create a dummy admin account even.
The hint is in the simple fact that the admin account is hidden from the standard welcome screen. In the years I've been using XP I've never needed to logon as the administrator. I see it more like an account that would be used on larger corporate networks etc. etc.
Try googling for some XP admin security advice.
Windows XP - Home User Self-Defence has some interesting reading. For the purposes of this thread, have a read of their
"rename the admin account" advice.
Remember though, while a little paranoia can be a healthy thing, too much just gets in the way and spoils things. Read a little and decide how tight you want or need things locked down.
The one thing they all appear to be unanimous about is that you really should have a strong password on your admin account, and you shouldn't be using it for general purpose stuff.-marko.