First, unplug everything from the front and back of your computer (be sure you can plug everything back in when you're all done; don't be afraid to take pictures, draw schematics, etc.). Start by dusting off the outside of the computer's case. Be sure to get floppy drives, disk drives, and lights.
Next, carefully remove the computer's case, but do NOT touch any electronics inside yet. Be sure to ground yourself before touching anything inside the computer, as static electricity can damage your computer's components. Now, take some sort of soft duster and remove/loosen any dust you see and can easily reach. Be sure to get around the computer's RAM, power supply, CPU heat sink/fan, video card, PCI slots, and any other slots, fans, heatsinks, etc.
Now use a vacuum cleaner to suck out all the dust that you can. Be careful not to bump or damage any computer parts while doing this. Also avoid sucking up any loose parts or wires (which shouldn't be there in the first place). Once everything is satisfactory, replace the case and use compressed air to blow out all the ports, front and back. Finally, plug all your wires back in and fire up your computer!
If your computer refuses to boot into Windows XP properly, you can boot into something called "safe mode." This will boot Windows with minimal resources running. This can be helpful so you can correct errors/applications that you cannot otherwise fix because the computer won't start properly.
Start with your computer turned off (recomended you print these instructions first). Now, step-by-step:
1. Press the power button on your computer to turn it on
2. As soon as you turn the computer on, begin pressing the F8 key on your keyboard every 1-2 seconds
3. A screen should come up with numerous boot options. Use the arrow keys to select "Safe Mode" and press Enter
4. Your computer should now continue booting into safe mode, and may ask you if you are sure you want to work in safe mode; Accept the message
5. Fix whatever problem you need to. Note that screen size & color may be reduced because safe mode uses minimal resources & drivers. Once finished, reboot into normal Windows!
There are several ways to download youtube videos. Here are several I am aware of:
Online Services
Firefox Add-ons
First you'll need to download the best web browser ever, Mozilla Firefox. This browser supports many great addons for many, many tasks. Next, go to https://addons.mozilla.org in firefox. Search for something such as "video download helper" or "download video" and you should get many results. My favorite is mediaconverter.org so I can download a video in a particular format for my computer or media player. Find a addon that you think you will like and will work, then install it into firefox. If you don't like it or would like to try another one, you can uninstall it and add a new one (or have mulitple addons for the same task). Now try downloading a youtube video! Good Luck!
If you don't already have firefox installed, download it and install it. Next, visit this page and download the ad-block plus add-on. Restart firefox and and it should ask you once what filter list to use (or subscribe to). Choose the one that applies to your language/location. That's it! Almost all ads will be blocked now. If you wish to turn it off temporarily, click its menu icon on the toolbar and disable it. You can also have it block custom items, images, etc. After always using this at home, I always notice all the annoying ads when I use the internet elsewhere! (Which is why I'm writing this now, as I am at my Grandma's) Happy browsing!