Extremely slow calculation of lookup formula
 

Excel 2003/SP2, running under Windows XP seems to take forever to calculate a
formula when it is copied to a new cell. My computer is a P4/1300 with 768MB
memory. Sure, it could use a replacement, but that's not presently in my
budget.

The problem occurred within the last day or so, and I can not figure out a
possible cause. Rebooting and running the error correction program for the
office installation had no effect. When pasting, the CPU usage jumps to 100%,
the task manager shows several instances of Excel as running.

The formula:

"=IF(J3763="","",IF(J3763<$A$1,VLOOKUP(A3763,Data1 ,HLOOKUP(J3763,Data1,2),FALSE),IF(J3763<$A$2,VLOOK UP(A3763,Data2,HLOOKUP(J3763,Data2,2),FALSE),IF(J3 763<$A$3,VLOOKUP(A3763,Data3,HLOOKUP(J3763,Data3,2 ),FALSE),VLOOKUP(A3763,Data4,HLOOKUP(J3763,Data4,2 ))))))"

is in a worksheet with data in cells A1 to K4500, and is supposed to
extract cell data from a different workbook.
Data1, 2, 3 and 4 are defined names for cell blocks in the external workbook.
Column J contains dates entered by a formula, cells A1 to A4 contain
reference dates.

Any help will be greatly appreciated
Rolf