One of the things about say, Windows is it tunes itself to the hardware on install. There could be other reasons but there might be a reason where a reload is required OR the BIOS is missing an update to support the I5 in full.
Details are missing so share a SPECCY of this PC for a check by me or members.
i just recently upgraded my cpu from an i3 to an i5. ive noticed that my computer runs a tad bit slower, is this because of bottleneck or could it be something else?