You can't tell a program to use only 25% of the CPU time, as this is at least partly controlled by the OS. You CAN place a variable loop time in the program though, which may be able to achieve something similar.
By placing a "Wait until ms multiple" in a while loop, and attaching a control to this, you can essentially vary the execution speed of the VI WHILE it's running. If you look at the current processor usage, you might be able to set the VI to use a certain % of the processor power. Please note that the results will be variable, and you might not be able to reach exactly your target. Additionally, as far as I know, LV is listed as a process, not each individual VI, so this whole approach will set the LAbVIEW use to the required amount. I'm assuming this will be more or less the same as setting the VI itself.
hope this helps
Shane.
Using LV 6.1 and 8.2.1 on W2k (SP4) and WXP (SP2)