Assuming you keep your virus protection up-to-date, you may not have a fatal virus, but you could have some files that came from your downloads that latch on to things and create the problems you are experiencing. You could also be sending these files to others while not malicious create problems. If you have a windows based machine, you could perform a reset to a date before the problems started or even scrub your machine completely and start over. The worst is you have several viruses though if you are still running that may not be fatal, but I would act fast.

to keep your system clean.

Nothing will slow down your system more than that, as long as your anti-virus program is up to date.
Never run more than one anti-virus program at a time. They will cancel each other out.

FileHippo.com is a safe and easy site to download free software.

Also try CCleaner, another excellent program you should have to keep your system clean.

Good Luck

How old is your PC? Eventually all PC's slow down due to registry corruption. The best potion is to back up your stuff and re-install OS.

Not joking by the way.

It's free.
It will clean the hard drive, and it has a registry cleaner as well. It gives you the option to backup the registry (which you want to do) before it fixes it.
After you run the cleaner, fix the registry, run the cleaner again, then de-frag the drive.

It should run almost like new.