It does sound like a temperature issue at first glance - those CPU temps do seem pretty high under low load.
There are a few things you can do, the first is obviously removing the side panel and having a good spring clean on the inside. A can of compressed air is usually handy for this job.
You can then remove the CPU heatsink and fan unit, remove the thermal paste from the old application with high concentrate isopropyl alcohol. Then apply a new application of thermal paste. The 'blob in the middle' method is probably the easiest to perform. Re-seat the heatsink and fan unit ensuring everything is seated correctly and you have good even pressure along the CPU. There are plenty of videos on Youtube if you're unsure.
How is the airflow through your case? If its an older OEM case then it may be difficult but this is a good guide to the basics of case airflow and is well worth the read:
the airflow kinda sucks.. 1 fan at the back (not the psu fan, it's a pretty small fan too) and then the airflow from the front is virtually non existent, with 3 tiny little holes to let useless amounts of air in. Fortunately, there's some okayish side vents
My dad recently took the case off to fix a problem with the power button and did a bit of a dust cleanup then, so there shouldn't be that much in there at the moment..
and with the xp version, we have some weird version that my dad says can't use SP3 or 2.. not sure why, might be some overseas version or something..
Unfortunately there's not a great deal you can do with a case that doesn't have the capacity for adding extra fans other than modifying the case. Are you able to fit an extra fan on the side vent you mentioned? This may help a little.
Are you able to remove the heatsink and fan unit and replace it using a new application of thermal paste? This would probably see the greatest decrease in temperature overall. An aftermarket cooler may also be a viable option if temps are still high.