I remember there were guides for Windows XP on all of the unnecessary services and things that you could turn off in a default install to make the system a bit more 'snappy'.
Is there such a guide for Windows 7? Suggestions?
By and large the rule of thumb for Windows 7 is: Don't Overtweak Anything. Seriously. Just leave the services on and don't try to turn off things you don't think you need. Microsoft Engineers have spent more time thinking about optimizing the system than you have, and the best you'll ever be able to do is save a few megabytes of memory at the cost of hard to diagnose technical issues down the road when you've forgotten what you turned off and why.
The system is quite good enough as it is; it's not 1998 any more.
Blackviper has a lots of good information on service configurations here.
I'm sure if you look hard enough you can always find services and features you can disable simply because you don't use them. However unless you really know what you are doing, you really shouldn't try. Especially on Windows 7. Its loads faster than any previous Windows OS and is already tweaked to perform very well on any relatively modern machine (even netbooks!).
As others already said, you shouldn't really tweak anything cause it already works quite well.
Only thing I do after installing it something like this:
Disable automatic reboot after system failure
Activate Verbose Status
Change the taskbar mouse hover time
I discovered AVG PC TUNEUP a few years ago and I have been happily using it with great success since. Not only does it gain you in-depth access to that dark hole called 'registry', it also makes backups of everything so if something goes awry you can reverse the process. Its user friendliness surpasses pretty much everything out there, and in combination with its sibling AVG Antivirus and REVO Uninstaller Pro there isn't much left to wish for. I strongly disagree with Mr. Factor Mystic; Win7 comes with a lot of useless garbage, at least for the average user, and out-of-the-box glory has not been seen so far with any of the stuff from Redmond VA. So, geek, 'superuser' or mortal, reserve the right to tweak your stuff to your taste, you may even enjoy the learning curve.
Just turn off what annoys you. For instance, I disable things that cause noticeable slowdowns (or used to back when I had hard drives in my systems) and/or reduce battery life:
-Superfetch (additional disk IO during normal use, no perceiveable performance benefit because I don't close applications anyway, and thrashes the disk when resuming from hibernate)
-Windows Search (indexer). Additional disk IO and often just doesn't work (certain files are unfindable until you refresh the index manually).
There's a few others, but these are the main culprits... whenever I set up a new Windows 7 system, I usually spend a few minutes wondering why the HDD LED is blinking like crazy when the system is idle... open Resource Monitor and bam, there are Superfetch and Search, thrashing away...