The only way to do this is to ensure that there is no malware or rogue hardware on or attached to your PC.
First you need to be sure that there is no rogue hardware attached to the PC. Unlikely if you have your own PC in a trusted environment but highly likely to occur when using Internet Cafe hardware, especially in certain countries.
Then, as @AthomSfere says, the safest way is to download a Live CD/DVD, burn to disk (could be a USB drive if you don't have a CD/DVD drive, just make sure you only burn the boot disk to it and make it read-only), boot to it and use that. You can then be 100% certain.
If that isn't an option, you should Google for the latest anti-malware tools, run a selection of them to be reasonably ensure that your PC is clean. The collection of software for doing that is pretty steep but below is a short selection of possibles.
- AVAST Free, Anti-Virus. Great tool, you can also go for the paid option with more features including an application white-listing mode which, when well configured, will make your system as secure as it is possible to be (bit of a pain to maintain though unless you only use a small set of software)
- Microsoft EMET, a Windows system hardening tool from Microsoft, make sure you follow the instructions for a basic configuration
- SuperAntiSpyware, check your system for unwelcome "guests". There is a portable version which I recommend
- Malwarebytes, similar to the above, worth running both. Can be uninstalled once run.
- You may want something to clean up outdated software or adware and the like.
- SlimClean - Straight-forwards to use, has a crowed-sourced knowledge base. Can be uninstalled after use
- BleachBit - recommended by security expert Bruce Schneier, is portable so no installation required, has a Linux version too.
- or the old school CCleaner may all be used.
Always use several tools in each category but only run one at a time - For Anti-Virus, only use one unless you are prepared to uninstall to use another.