1. Making Registry changes to speed up a computer : In some very specific cases changing the registary can speed up your computer a little bit. Sometimes. But by & large, spelunkinng through your Registary looking for a killer bit that'll boost your speed by 10% is a fool's game.
   If you do find hack that'll speed up your computer by 10%, you'll never even notice the difference. It takes a speed-up of 20% or so to be noticable- and even a 30% goin won't seem like much after a day or two. On the other hand, if that speed-up hack makes your computer unstable, belive me, you will notice. Right between the eyes you'll notice.

2. Using any registry cleanup utility : Ther's one born every minute. Software companies have made millions & millions of dollars offering programs that scan & clean your registary entries. I've never seen a single registary cleaner that was both safe & effective. Sure, your registary gets clogged up with useless entries. Does that make your machine slower? No "Cleaning" little known registary entries can make your machine roll over & die. You're better off using a sledgehammer.

3. Disabling Windows processes : I've seen a dozen reason for disabling the Window programs that automatically launch when windows starts. (You can see which processes are running on your computer - execept the really stealthy ones - by pressing Ctrl+Alt+Del & clicking Processes in the Window task Manager) None of the reason I've seen for disablig processs manualyy makes any sense. Years ago, a Window process called UPnP opened a huge security hole in Window XP. Microsoft fixed the hole, eventually, & for a while it made sense to disable the UPnP process.
   If you have  renegade processes, they're symptomatic of another problem - generally a worm, spyware, or some other form of malware. You should treat the problem by zapping out the offending software instead of attacking things placemeal by manually disabling a runnig process that you don't understand.

4. Assigning fancy file access permission : Yes, I know you can fine-tune folder & file permissions, even with Window XP Home, So you can allow certain people to see your files, keep others out, & make others dance like bears before the data comes up. But for most people, it isn't worth the effort. Why? If you really need to set up sophisticated file access permissions - so that 1 person can read the file & update others file - & you aren't willing to use passwords, you need to bite the bullet & install a server, Window XP Pro, & Active Directory. The half- fast approaches to ristricting access in Window XP Home are doomed to failure simply because the tools aren't all there.

5. Doing anything involving DOS : Okay, there are exception. I even talk about one of them. But if you aren't conversant with DOS - & few of us are after all these years - you can almost always find better way to accomplish what you want by using Windows alone or one of the nearly infinte number of free utilities.

6. Making parental control software & modification : Doomed to failure, no matter how hard you try . Windows does't have the tools to properly implement parental control, & the world on the Web changes much too fast. Don't even bother.