The short answer is that a PC cannot "become slower". However, the PC's workload can most certainly have increased without you even knowing about it!

How could that have happened, when I haven't really installed that many programs ?
Well, many small programs install them selves and you wont ever see it - it's what's commonly known as spyware.

So tell me, how did this spyware find its way to my PC ?
This could have happened in a number of different ways:
- ActiveX controls are programs designed to run on your PC, whenever you visit a site that need the particular ActiveX control, to properly display its content. Many websites use this technology for perfectly safe purposes. But an ActiveX control can also install a small program on your PC, that is in fact a piece of spyware.
- Downloading MP3 files is another way of letting malicious software slip into your PC. The spyware can be embedded in such a file and it installs itself on your PC without any warning.
- Freeware is software that can be downloaded and used freely from the internet, but sometimes there is a catch. Embedded in the freeware program there can also be a piece of spyware. Usually this will be mentioned in the User Agreement provided with the software. During installation you then have to click "I Agree", in order for the installation to complete. The problem is that you will normally never give this a second thought, because you have to click "I Agree" with almost any software you install. Worse is that by clicking "I Agree" you have allowed the supplier of the program to install the spyware on your PC!
- Toolbars in your web browser are not always just toolbars. If a new toolbar suddenly appears in your browser, there is a good chance that it actually contains spyware. If a website offers a free toolbar, I highly recommend that you do a search on it, before installing it.

These are just some of the common sources of spyware - the next question is:

What exactly does the spyware do on my PC ?
The purpose of a spyware programs vary a lot. Some of it more of an annoyance, than an actual threat. But there is also really dangerous spyware out there.
Lots of spyware gathers information and sends it back somewhere for a particular use:
- Surfing habits is useful for companies when they target ads for their products - which sites does the user look at, what kind of products does the user search for, etc. The information can be used to make targeted spam emails.
- Emails you say? Yes, spyware could also detect your email address and relay it to someone else.
- Sensitive information like account numbers and passwords are also at risk. Some spyware will scan your hard drive for a particular piece of information.
- Keystroke detectors with the sole purpose of finding usernames and passwords.

You may also experience an increase in pop up ads. That is, windows with ads which automatically opens up in your browser, when you are surfing the web.

How do I know if my PC is infected with spyware ?
There are several symptoms of a spyware infection. I have already mentioned some of them in this article (increase in pop ups, unexplained toolbars, increase in spam emails).
Another symptom is if your PC seems to have become slower. This will occur if you keep going for too long without any spyware protection. The amount of undetected spyware on your PC increases and since all of these programs run in the background, they all require system resources and eventually to an extent, where it feels like your PC can no longer keep up. If you just bought a high end PC with a fast processor and lots of RAM, it will take while before you will notice a performance hit (if it will ever come), but this only makes it even more important to protect yourself.

I believe that if you surf the internet on a regular basis and over a broad range of sites, you WILL eventually get spyware, if you are not protected. In fact, I will not be surprised if 90+% of ANY PC with internet access and without spyware protection is infected to some degree.

The Cure
You can find a link to a very effective anti spyware program on my website www.fixmypcnow.net. The program can be downloaded so you can try a scan for free (the program is not freeware though).

I decided to scan my PC and I found lots of spyware. It has been removed now, but my PC acts "funny". Why ?
For obvious reasons spyware programs do not have an uninstall option. You may be able to get rid of it, but there is a chance that errors have appeared in the Windows Registry. If you experience errors when using your computer, I will recommend that you run a scan of the registry in order to determine if something is wrong - especially if you are recovering from a spyware infection.

However, spyware isn't necessarily the cause here. When you uninstall a program, the uninstall wizard is supposed to "clean up" after the program and this doesn't always happen properly. This also leads to errors in the Windows Registry, causing Windows to behave unexpectedly. If you have used your PC for a very long time and have installed/uninstalled lots of programs, there is a good chance that there are errors in the registry.
On my website www.fixmypcnow.net you can find a link to a powerful tool, which safely fixes these errors.