MSI.DLL MISSING...CAN'T ACCESS ADD/REMOVE PROGRAMS

Help, when I signon..I get the message msi.dll file missing...I've tried reinstalling windows installer and enabling windows installer to no avail...(I have windows xp pro) Dell dimension E510..I've tried re-installing IE 7 and still getting the message...also can't access itunes and several icons...can't right click...the word 'file' is also missing from my top toolbar....(don't know where it went.)....Can't open Add-Remove Programs ....I'm in such a mess...is it time for a new computer? ( 2 years old!) or just some good tech help?

Thanks,

zmdqtxd