The term registry bloat means the Windows 7 (or any other Microsoft Windows Operating Systems) registry is filled with obsolete or unnecessary data those are useless on that machine and will only slow down the machine, interfering with normal operations or cause malfunctions or crash the programs.
Many forms of malware such as spyware and Trojan horses can also cause this registry bloat by adding additional registry keys for their own use that have no value to the normal operation of your Microsoft Windows 7 based machine and, in fact, may exist in order only to slow your machine down. 
These can infect your machine through a number of sources, most often via emails or bad website’s malicious programs get installed on your computer for a number of purposes, not to say; for their own benefit. Moreover, even if an anti-virus can get these machines and can prevent more damage but virtually all anti-virus software on the market today can not do anything to the registry of your computer.
For this reason, the registry bloat can be a malignant problem that may have entered in your machine for a long period of time that may go unnoticed until it has already reached a point where it is impossible to easily eliminate using conventional means.
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Why they do it:
They may have the intention to steal your passwords or keep an eye over your online activities. Not to say, these malwares will send the “update” of your activities to their servers.
Prevention:
Prevention is better than cure. Avoid clicking fishy looking advertisements (or you may modify your HOSTS file to add more security). We have discussed how to edit the hosts file in Windows 7 earlier, you can read it if you are not sure how to edit Windows HOSTS file.
Java is the main medium to get these malwares while browsing. You might think to inactivate Java, Active X for normal browsing. Other than security, it will save your bandwidth and time (by quick loading of the pages) too.
There is a quick method to block Advertisements when you are browsing with Firefox; a simple Add on, named Adblock does the job easily.
However, many websites are just surviving on revenue from good Advertisements from Google Adsense or Adbrite: both of them runs a manual check before considering to give ads for their clients. It is up to you, whether you will block them for all websites or not.
If your registry is already got bloated, you may consider a fresh format to prevent further damage to your valuable data after taking a manual backup of the data. We do not believe or rely on Registry cleaners that much. infact, some of these Registry cleaners are itself a bloatware!
Future trend: Internet Explorer 9 may come with integrated Ad-blocking feature. Obviously, it will take care more for the safety of the Windows 7 users.
The other general security measures for your Windows 7 has already been mentioned in this website before. They are applicable for this topic too, as other ways of prevention.
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