Hello and welcome to Bleepingcomputer.
This sounds like a good time for summer cleaning....the inside of the computer that is.
A slow system can be contributed to things getting a tad too warm and the system will start to slow down a bit over time and eventually will cause crashes.
Disconnect the power supply and open the computer and clean out any dust you find inside that case, be safe, if you touch anything inside make sure your grounded to the computer case while your touching anything inside.
Purchasing a can of compressed air is great for blowing dust out of hard to reach places, just do not let the fans spin from the force of the air, use a pencil to stop from spinning and you might want to try putting a new bead of thermal paste/grease between the processor and heat sink while you're at it.
Once you do that, you might find things will have a tendency to speed back up again.
Edited by MrBruce1959, 27 June 2010 - 12:52 AM.