Computer running slow

i have a hp a1230n pc with a amd athlon proceesor.i upgraded the pc with 4 gigs of pc3200 ram and had to replace the cpu fan in order to change cpu.i used a thermaltake v1 hsf.my computer uses a 300w power supply.i then upgraded the cpu to a socket 939 amd athlon 64 x2 4800 processor.i upgraded the bios to v 3.15 which i got from hp.i used cpu-z to make sure if my pc recognizes the processor which it does.the problem is now my pc takes longer to boot up and runs slower.i checked for viruses,adware,malware and also fragmented my hard drive but nothing seems to help.if it is due to graphics i use a radeon xtasy x300 pci-e graphics card.i would appreciate any help.

when i click system in control panel it is showing the right processor running at 2.39ghz.

wellll,it could be ram timings/voltage settings especially if you have 4x1 rather than 2x2???,the last time that i had a computer run slooow on me was when the psu was giving up the ghost,300w seems a little light,, can you try a 400-450w psu,that would at least eliminate one problem..
The other item to keep in mind is that sometimes when oem's design a computer there is not a lot of room.electronically speaking,in the design for serious or sometimes any upgrades,,hmmm..