Computer always searching for Wifi connection; slows computer

Hello,

I recently bought my mac (21.5 inch) with a wireless mouse and keyboard. I am running the internet through a wireless cabel modem which works fine on my other computer (which is a PC laptop). Also, I use firefox as a browser.

My problem is that recently my internet connection is extremely slow. Often, while typing, no characters are inputted and the computer seems to freeze for a few seconds, then enters it in all at once. So, while typing this note, I typed at my normal speed but saw the characters being inputted chunks at a time. Also, when scrolling with my mouse pages seem to freeze and then jump down suddenly to the bottom. It seemed to me that there might be a problem with the keyboard or mouse, but for both to lose battery power at the same time seemed strange. I checked the internet connection at the top and I found that the computer is always (and I stress ALWAYS) looking for a connection, even when I'm online and searching/downloading something. So my questions are: why would this be? And, if this is the problem, how can I fix it?

Thanks very much for any help you can give me,

Odessa