There's a confusing array of free tools available. Many contain bad things - malware, viruses, hijackers. The below are all great tools you can trust and try for free. Buy them if you like them to insure yourself against further attack. Also use windows system restore found at Start Programs, Accessories, System Tools, System Restore. It doesn't destroy your data, it simply resets your system settings to before the infection. You may need to press F8 during the boot and choose safe mode with networking if your computer is behaving too badly.
Antivirus Program, try for free (15 Days) The free trial will remove most viruses, uninstall your old antivirus program first using your Control Panel. The most popular antivirus programs are targeted by virus makers. I mean they go out of their way to work around them and turn them off. I have removed viruses and spyware from "protected computers", including CA, Trend Micro, Norton and McAfee. I like VIPRE because I can install it on computers that refuse to install the other antiviruses, it doesn't slow your computer down and it's a good price at US$29.95 for 1 year. cheaper if you get 3 years.
Synchronise Automatically with your external hard drive. The cost of storage and external hard drives leaves no excuse not to backup your data regularly. I love handy backup for its ability to set and forget. It will backup, restore or synchronise at a scheduled time. It will email you each time your backup runs so you're aware if there is something wrong or at ease because of a successful completion. Try Handy Backup 6.0 free for 30 days
To assist you in keeping your computer infection free and remove current infections.
Malwarebytes' Anti-Malware Malwarebytes' Anti-Malware is an anti-malware application that can thoroughly remove even the most advanced malware. It includes a number of features, including a built in protection monitor that blocks malicious processes before they even start. You can download it for free from their site http://www.malwarebytes.org/index.php
As you use your computer it collects all sorts of crap from the 'net. CCleaner is a freeware system optimization, privacy and cleaning tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware! :)
Hijack this is a bit trickier to use, but a very powerful tool. Find it at http://download.cnet.com/Trend-Micro-HijackThis/3000-8022_4-10227353.html Be careful using it, look for 04 startup programs that you don't recognise. Look for 02 BHO (Browser Helper Objects) that don't belong.
Acronis Disk Director Suite gives you the power to do what you like with your hard drive. Create a backup (an exact copy of your hard drive) and a boot CD which makes it quick and easy to get going if your hard drive crashes, delete and copy partitions. The free trial will help you get started but you will need to purchase a licence to use the recovery features.
Acronis True Image has a non-stop backup feature now.
Consider getting us to setup a RAID backup system where you have a 2 hard drives, the usual 1 and the second is a mirror of the first for easy recovery after a disaster.