I have a desktop computer. It boots up O>K but programmes will not open or respond properly until it has been switched on for at least one hour. ANY ADVISE?

Check the RAM still problem exists,replace IDE cable or SATA cable , still there is same problem then defrag hard drive (before that create backup of essential files), uninstall unnecessary programs and antivirus , still in trouble reinstall OS with xpsp3 or better version , still there prob exist then replace hard disk and try to buy hard disk with 7200 rpm or higher , 10000 rpm is the better one and don't forget to update the RAM too, if it is necessary

You may want to try reset to a previous time when it worked. If not, do a back up and factory reset. If it is still doing it it, it is a virus.

Run a virus scan, clear out your cookies, empty your recycle/trash bin/file, and if none of that works try maybe calling someone over from your local BestBuy or any other electronics store to take a look at it.