Ok, so I'm first? Let's go, then.
First off, you should consider buying an SSD for installing your OS, this will make you happy every day. Install Windows 7, check automatic updates - after completion this is the first thing you'll want to do - update.
Next, disable the "remote registry service". Press the Windows key + R, type in "services.msc" and set this service to "disabled". Turning off the "Remote differential compression" under "Programs and Features" -> "Install/uninstall Windows Programs" will enhance speed (it did for me, at least).
Then, disable System Restore an ALL drives (go to the control panel to do so).
Basic installation awareness: Do not opt to participate in any "improvement programs" nor should you allow any automatic "error reporting". ALWAYS check every option you selected. If you are offered a "Custom Install", DO it.
Then you absolutely need to make your browsing more secure! I recommend Firefox, download it and get the following vital add-ons:
AdBlockPlus, Ghostery, HTTPS Everywhere and NoScript. Set it as your default browser and do not use Internet Explorer, if you can.
Get your AV! I personally recommend AVG, their free version is OK, however you really should invest the few extra bucks for the paid Internet Security version. Update it and allow "in the cloud" verification. Then perform a full scan, be careful to enable "scan for PUPs", "enable thorough scanning" and "enhanced set of PUPs" in the advanced options under "Scans".
Malwarebytes Anti-Malware is your friend out there. Install it and don't choose the free PRO-trial, unless you want that extra protection. I can't tell you if this will interfere with your AV, though - never tried it.
Spybot S&D is the second tool of my choice. Download version 1.6.2 (and this version only!), it can "immunize" your hosts-file, blocking the most atrocious & well-known badsites from being accessed. It also is a little more thorough than Malwarebytes, if it can't detect this and that. Set it to scan "all file sets". Forget Microsoft Security Essentials, bottom-of-the-line, IMO.
Install Priform's "Ccleaner" to rid your system of junk, one of the most popular downloads out there
Check it out!
Set Windows Backup to automatically run (best on an external disk, if available) and let it "include a system image". If you have a blank DVD handy and you don't have an original Win7 disc, use the "Create a system repair disc" feature now. I personally use "DriveImage XML", a wonderful lightweight backup program, should you need an alternative.
After that, install all programs that you need (duh!), the FlashPlayer for your browser should ONLY be downloaded from
www.adobe.com ! Don't klick on anything that says "Klick! "Klick me!" "Free offer! "Yu win 5 milion dolarsc!" Be rational and alert while on the internet. You know that, of course.
Run regular scans with your AV and the anti-malware programs, and if you're certain your system is clean, back it up now!
Those are my personal "brandboy" recommendations
I'm sure others may disagree, but you have the advantage of getting a variety of (hopefully) good alternatives. Hang in there, keakar!