A few things to try:

Look at the signal on the RB6/7 lines.  Make sure there isn't any
ringing on the lines.

Look at the voltage on the VPP pin.  Make sure it's going high
enough.  Make sure that the voltage is propagating through to
the pin itself.

Can you pull the crystal so the program can't start executing?

Mike H.

>Hi Everyone,
>After doing more testing on my ICSP circuit I strongly believe that the
>PIC isn't going into programming mode once VIHH is applied to MCLR. It
>simply goes along and executes code as normal. Has anyone seen similar
>behaviour?
>
>The pic (16f877) has a max232 connected to its Tx and Rx lines, is
>running a 20MHz crystal with 2 15pf caps.
>
>It also has a 4x20 LCD screen connected to RD1-7, in 4 bit mode.
>
>Regards,
>Daren
>
>
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