Good place to start, re RAM. Could well be the problem - but then you have to identify why your memory which was once adequate has suddenly become inadequate.
Trotting out the usual stuff - but it could be youve got loads of resident programs running (non techie way of telling is looking at the amount of icons youve got in the application bar - techies get nervous if there is anything but the clock their
). Or it could be that you have a virus or some malware.
Also stuff like outlook and outlook express can start to take ages to run if youve been messing around with newsgroups - so keep those cleared down - but if everything is running slowly then thats probably not related.
Could be that you are running out of harddisk space on your system drive
Sometimes its because the hard disk is starting to fail, or that its heavily fragmented (which exacerbates performance problems when ram is at a premium)
Well there are some ideas anyhow - its quite a broad subject to give advice on.