I'll usually run AutoRuns from SysInternals to check the startup programs. I'll also try running MalwareBytes Threat Scan, and perhaps a defrag. Many times when people complain their computer is slow I'll find that they're running more than one antivirus, and that slows things down a lot!
If the computer still seems slow, sometimes a format and reload is best. If you prepare for this in advance, a full format and reload doesn't have to be a very long process at all. You can make customized CDs that make re-installing Windows faster and easier. I'm going to make a tutorial later on this, but here's my basic workflow:
1) Multi-OEM Project (At MyDigitalLife.info) - You know how if you install Windows from a cd that came with the computer usually when you're done it will be already have the product key entered and Windows activated for you? Yeah, this project lets you make a universal cd for that which will work with over 100 different laptop manufacturers. As long as you're installing the same version of Windows that's on the sticker, it will be pre-activated and apply the manufacturer brand theme. It's pretty cool, you'll never have to ask someone "Do you have the cds that came with your computer?" again!
2) nLite (nLiteOS.com) - Allows you to slipstream Windows Updates into the installation cd. This way you won't end up with tons of windows updates to install after installing Windows. This easily saves you hours of time! You can also automate some of the setup prompts, but I usually don't for multiple reasons.
3) RyanVM's Switchless Installers (RyanVM.net) - Hopefully you won't need to use this too often. Ryan made switchless installers that helped slipstream updates that you normally wouldn't be able to for Windows XP (specifically the .NET Framework). They're not really applicable for Windows Vista and up, but his site and the forums have some pretty cool stuff to check out.
4) DriverPacks (DriverPacks.net) - This is a huge time saver. It's a full collection of nearly every driver imaginable. You can slipstream them into your Windows Installation so that when you are done installing Windows, all of your device drivers will already be loaded for you! Or, you can use DriverPack Solution (
drp.su) to install and update drivers on an existing Windows Installation.
Using these steps, you can make CDs that make the whole Windows installation process faster, automating product keys and activation, Windows Updates, and Driver Installation.