This is a can of worms.
I don't know that you can overclock that system. A lot of your PC companies (Dell, Compaq, Gateway, etc.) will supply a PC w/ a motherboard that has no jumpers or BIOS ability to change the CPU voltage, frequency, multiplier and so on. Maybe someone here can help you on that part.
Regarding your games being slower, are you sure you have the latest and greatest drivers for all of your hardware (specifically video)? If Windows loaded a default driver from it's d-base it may be outdated. Check the manufacturer's website for some reference drivers. These typically aren't supported by the manufacturer if you didn't get the item directly from them, but sometimes are quite a bit ahead of the supplied files.
There are a zillion things you could check, but I'd make sure the system properties are all tweaked out (UDMA enabled on drives, refresh rate for video adapter/monitor, etc.). It's possible you had tweaked something before and forgot about it.
Also, check your version of DirectX. 8.1 is the latest and greatest.
Last, but not least, you could fool around in your BIOS if you're comfortable and tweak it out as well. If you have an ATA/100 hard drive, make sure that feature is Enabled, ensure AGP is set to 4X if that's what your card supports, etc. Don't touch the BIOS unless you're comfortable doing so.
Hope this helps. I just solved a problem I was having w/ an XP install that was related to my DIMMs being 2 different speeds
