Hi Karien,
Would be interesting to see when you go to the old Dos-promt and give in the command: chkdsk and post the read outs for that on some consequent moments. Just to see what the hardware gives away. If the OS is cleansed, and the problem remains then we gonna think in that direction.
First things first so download startdreck from http://www.niksoft.at/download/startdreck.htm Fire up this nice German proggie ((406.585 Bytes) md5: cf15b20807e52446503ab2742e5acf55 ) and post the findings here. Download Crap Scanner also from here: http://www.niksoft.at/download/crap.htm, but do not fire it yet.
polonus