Pink-
I think your problem is probably more on the software you are trying to run requires more machine than the one you are running.
here are the instructions for turning off start up programs you do not need:
Here are simple steps to Disable Programs at Startup.
1. Click on Start
2. From the Start Menu Options choose Run
3. Type msconfig without quotes and click okay. This will bring up System Configuration Utility
4. System Configuration Utility >> Click the tab called Startup
6. Now you can uncheck any programs that you do not want running upon startup
7. Once you are done unchecking Click Okay and you will be prompted to restart your computer
just wanted to add that you shouldn't be afraid of unchecking something u want, because it can always be re-checked the next time around.
To find out how much ram you have, go start-->right click on "my computer" select properties, and under the general tab it will tell you how much you have.
Reformatting and reinstalling windows is by far the best solution. In fact it's considered good PC maintenance to do this every 6 month to keep the computer clean.
Do you have msft office? if so, get rid of it. It's overly graphical and bogs down the system. Get open office from openoffice.org It does nearly everything Msft Office does without the crap and is fully compatible with Msft office documents.
use mozilla firefox instead of IE. Doing so reduced your chances of spyware infections.
For anti virus, the same thing applies. Use one that's not so darn graphical. Norton is horribly graphical and takes lots of system resources. A virus scan should scan for viruses, not look pretty. When ur subscription is up, get AVG free anti virus from download.com
Another thing u can do to improve performance is turn off all the graphical features of windows. You basically transform the computer to look like classic windows. To do this, go start-->right click my computer-->properties-->advanced-->performance settings-->select "adjust for best performance" option. It will make things look basically like windows 98, but it will speed up the computer.
As for updating windows, going from XP to vista is never a good idea. Vista sucks, it's overly graphical, unstable, and requires wwaayy more system resources than XP. I am a tech support agent for VIPdesk and Vista is the biggest PITA I've ever seen. I curse it on a daily basis and it's the reason I bought a mac. In terms of windows updates, it's a good idea to get the service packs even though they will slow down the computer, but I try to keep the optional updates not installed unless needed.
If you want quick and easy, install more ram and reformat. Those two things are your best options. The other things are tips and tricks, but don't hold a candle to reformatting and more ram. I've personally never seen a performance increase from defraging a computer, but I suppose others have.