There are many possibilities ...assuming the PC did actually once appear to be "fast"
1) Identify what programs are launched behind the scene upon startup ...identify them and know either what they do or the company behind them ...try turning off unidentified ones so they don't run (one at a time).
2) Many find there are several antivirus/spyware programs running at the same time ...and Norton, a good company, can really slow up things at times. ...use one antivirus/spyware app
3) Always expect new applications to take up more memory... they know memory is more available... can you upgrade your memory? ...if anything is graphics intensive, max out your machine on memory
4)Older machines with less memory will use the hard drive as a temporary memory swap if needed. Hard drives are slower than memory chips, if the drive is full, you can bet the data is stored in bits and pieces (fragments) which the drive has to find and reassemble when needed... this slows things down ...bottom line: a: be aware if your hard drive is filling up... b: unfragment the drive once in a while
5) Display settings that attempt to give HiDef imagery, 3D, etc ...all go through more processing to deliver ...and if you're gaming on it, definitely look into better graphics cards
6)Understand, when on the internet, snippets of data are exchanged from one machine to another... these "packets" get sent to a machine that doesn't know what it's getting but during the handshake, the machines verify the packet was complete and the same as what was intended to be send.... if it wasn't correct, it repeats the attempt until it is correct. Did you know you are actually going through many many machines to get where yer talking to? ... now, rethink the life of that packet.... whew! and any interference will just make it worse.
7) Everything mentioned above is simply a thought to consider... and definitely not all possibilities... and the pc is trying to do it all at what appears to be "real time" ... especially if you have more than one app running ( listening to music while working a spread sheet ... is that music from the internet? ahhhhh browser, bandwidth traffic, ...) ...things can sneak up in there... so while trouble shooting, just be aware of what's running