Pls HELP-Installed Ext HD now nothing works right!

I have a Dell Inspiron 8600 1-1/2 yrs. old, 1.6 GHz, 1024Mb RAM w/60G HD
(NTFS). I've recently installed a Seagate 300G external HD (Fat32) w/USB & a
500G Western Digital (NTFS) also USB. I use the Seagate for backup only &
the WD for files, photos, music, etc. The Seagate has been online for a
couple of months & everything has been fine. After I installed the WD,
everything has slowed waaaaay down & software keeps crashing (Paint Shop Pro,
Explorer, Word & others). My computer hangs and sometimes goes on but
sometimes whatever program is running "encounters an error & has to close".
I'm NOT real savy about all this stuff so please be patient with me but
please help!!!