Sluggish Performance

I've recently had a new PSU installed and since then had my computer perform sporadically slower than it did previously.

For example, when windows first loads if I move my mouse the mouse lags. This same preoblem happens if I open an application or just intermittenty whilst using a program. If I play audio or video it will occasionally skip.

I've alreday had the problem looked at by the hardware guys here: **Link Removed **and they concluded that my system is running at the correct temperature etc.

I've had it confirmed that there is no malware here: ** Link Removed** and was recommended I try this forum too.

I am running Windows XP Home Edition on an Aries Pc System, AND Athlon 64 Processor 2.4GHz with 1GB of RAM.

Any help would be hugely appreciated as I am getting desperate now.