Eugene's observations

The most frequent ailment I encounter currently is computer speed restriction.  This is most often caused by the owner downloading (perhaps inadvertently) programs that are bundled with something else.  The problem often starts with a new off-the-shelf computer; they are almost all stuffed with unwanted commercial software trials that purport to add value to your purchase.  By coincidence they also boost the seller's profits through partnership deals.

Another common route of entry is for the parasitic software to come bundled with the desirable.  Acrobat Reader is a good example: search Google for a download location and several sites are presented, all of which come with a payload of partner products (trojans). Even Adobe's own download contains McAfee Security Scan unless you opt out; so it is essential to read through any windows that appear to ensure you are not acceding to install potentially dangerous and unwanted software such as search toolbars or worse, malware ridden scams.   

If you do get caught there are some automated tools that can help rid you of many problems.  Try the following in order and stop as soon as the problem is resolved as these get more aggressive further down the list.  I recommend taking a full image backup first in case things go from bad to worse.

1. Adwcleaner removes adware, browser hijackers & toolbars. Free from
   https://toolslib.net/downloads/viewdownload/1-adwcleaner/
2. DeCrapifier uninstalls known rubbish.  http://pcdecrapifier.com/ 
3. Remove Rootkits by getting hold of
   http://usa.kaspersky.com/downloads/TDSSKiller 
4. Junkware Removal Tool deals with the common Ask, Babylon, Conduit, & MyWebSearch infections.  Disconnect form Internet & pause regular virus scanners before running this. Be prepared to reinstall Chrome after.  Free from
   http://www.bleepingcomputer.com/download/junkware-removal-tool/
5. RogueKiller  will stop suspect processes, services, DLL registrations, repair HOSTS and master boot records.  Best to make a system restore point before running this one.  Free from
   http://www.adlice.com/softwares/roguekiller/
6. BitDefender adware removal tool downloads from
   www.bitdefender.com/media/html/adware-removal-tool-for-pc

Most of the above run in under 10 minutes as they target specific problems.  For complete scans of the disk taking up to an hour to remove malware consider the free
http://www.malwarebytes.org/mbam.php  and
http://www.superantispyware.com/index.html

Irrespective of what's discussed above it is essential to install a paid for Internet security suite.  And more than that, make sure all programs including Windows itself are kept at the latest versions.  This can be easily accomplished by running Ninite as described at
https://ninite.com/updater/  and available from me.