could be the age of the pc, but it's usually just 'clutter'.
As Howard says, run a full scan (you can get avast for free if you have nothing). Also check what the pc's running when it starts-up as that'll all be on the go in the background, taking up time and reducing processing speed.
and if you don't do it regularly, then clear your temp internet files and history etc.
Good luck
Fllowings are some solutions to speed ur computer:
1. Delete unnecessary programs and bad files.
2. Repair the corrupt data.
3. Install a new OS.
4. Upgrade hardware.
Everyone gets this. Do respond and follow the advice given above. Often as programmes get larger lack of memory can be an issue. That's fairly cheap, ensure you have the maximum installed if the 'clean ups' fail. And check you are short of free hard drive space. That can have a major affect too.