with a 2g celeron fitting 2 gb of ram could well slow your system down - not speed it up
a celeron can only address 2Gb and that includes bios, video, audio, PnP and all the other little chunks of rom and ram
windows creates a swap file at least equal to your ram - generally bigger.
do your perfectly adequate PC now has to create and manage a chunk of disk 8x bigger than it was
on an older machine - it's reasonable to expect slower components, smaller disk and lots more baggage
the processor load is probably a combination of increased disk activity combined with a full disk (windows tends to use % of free space) so all the temp files will have to be deleted and re-created constantly to maintain that presentage..
seriously 1Gb is more than enough - consider e-baying 1 of the sticks - you don't need or want it