Are there any default services that I should turn off to help secure my machine?

A great guide to the Windows XP services can be found here:

http://www.blackviper.com/WinXP/servicecfg.htm

According to the site, these are the services you should disable to keep your system more secure, if you don't need them for anything:

Messenger (as mentioned above)
NetMeeting Remote Desktop Sharing
Remote Registry Service
Routing and Remote Service
Server
Telnet (note: this does not affect the Telnet program that comes with Windows, it just prevents people from using Telnet to connect to your machine)
Terminal Services

The real key to computer security with a Windows XP PC is to do these four things on a regular basis:

a. Download the latest Windows Updates for your system
b. Install an antivirus program such as Norton Antivirus and download the latest virus definitions and program updates for it
c. Install a software firewall such as Norton Internet Security/Personal Firewall (don't rely on the built-in Windows XP firewall) and download the latest security/program updates for it
d. Scan for and remove any adware/spyware programs on your system with one or more anti-adware/spyware programs such as Ad-aware and Spybot: Search and Destroy

One more thing: keep an eye on what things you transfer onto your computer from removable media and what you download from the Internet/file-sharing programs. Use common sense and don't go downloading things that could be a security risk. For example, hack and cheat programs developed for Blizzard Entertainment computer games (very popular ones) often come "packaged" with trojan viruses that can steal information from your computer, such as your account name and password for Battle.net (a common problem). Another example: downloading pirated versions of full-version software programs off of Kazaa, or anything off of Kazaa for that matter is a good way to get a virus.

Sorry about my little lecture there, but when it comes to answering questions about computer security, I think people need to know the whole story.

If you get those "Messenger Service" windows with all that SPAM in them, you should turn off "Messenger" in services in the administative tools. Turning it off will not affect anything else. It'll just stop those messages. Also, make sure you disable it so it wont start up again.